Hi, welcome to RO!

With classical the most economical way to do it would be to get the ensemble together and record live. If your personal facility isn't up to snuff, renting time in a local commercial studio should be fine. You'll benefit from the good room and equipment, as well as experienced engineer. Having them play together should give a good feel to the recording while also minimizing billable hours for the musicians and studio.

As to your questions specifically:

1. ask the musicians what their rate is, rates vary widely based on location, and the person's experience and schedule. Ideally they have experience in a studio.
2. without a click you will need to supply background tracks for the person to follow. This is where recording the ensemble can be advantageous. Following along with backup tracks that aren't recorded to click can be tricky depending on how much tempo druft there is. The other option would be to deliver sheet music to them, and have someone be the "lead" and everyone else who's overdubbing after follow them. This would be if the garage band tracks are not used as the reference track.
3. giving them the demo tracks, and sheer music as soon as is practical will help the musicians get ready. Delivering along with that any sort of notes about feel, or other production notes will help them dial in ahead of time.

Welcome to recording classical music! Many tech savvy players have really good home equipment now, with mics tailored to their instruments in nice sounding rooms. I’ve done quite a bit of isolated recordings during covid, and they’ve carried on. The click question is vital, especially for string players. I’ve done all sorts but the killer is ‘ensemble’. They need two things. A timing reference, and for the weirder contemporary stuff picking up the tempo from slow speaking instruments is so difficult. They also need the other players contribution to play the right pitch. If you have a string quartet in the studio, they all play around the correct pitch, not exactly on it. That produces that bigger chorussy, ensemble sound. The violin might play the Eb, and the viola adjusts their tuning to match it, but so does the cello, and the viola might shift a tiny bit. The violin player detects it and alters theirs and so on. A constant movement. You have a problem. You can’t do this separately, so you will probably have to create a guide, containing all parts that they can play to, but it wont be as good as the real players, because the pitch is fixed. Until they record, you don’t have anything real to play to. It produces a little sterility to their performance. One guy who plays very good viola and violin asks me for the chance if we have time, to re-record his parts once some real stuff has been recorded. 
my usual way of working is to provide them with two mixes. One has click left, a blend of samples playing the piece right without the track being recorded, so no violin if that’s what the outsourced player is doing, and one with that included. Problems are that often instead of using a balance control to turn down the click, they half pull off the headphones and the damn click leaks into the mic. You also get the players asking for different things. It’s a challenge. If you can get them together that is the best situation, no question. If you cannot, then you need to assess their technical ability before starting.you may well be very surprised. I have had my violin/viola playing contributor redo one of the other musicians parts because while they were a good player, they really were not, recording on their own. They needed the other people and their playing was boring and stilted. Some cannot play to a click. They just do not have that ability. They can, however play to the track with a click. They’re all different. 
I can’t comment on prices as in the UK, some are very union oriented, others less so. As the dollar and pound are the same now, I could get people for £50/$50 for around an hour of their time. They will not spend hours rehearsing though, they’ll just play what you give them. One snag is that the playing of some sections will NOT be together, so what I do is spot these twiddly bits, and send them examples of how it goes at say, bar 57, so they know how to play it, that way, they all play it the same with minimal time wasting on bits whee ensemble doesn’t matter so much.

I got a bit carried away with remote recording, so a few comments on recording classical material. Your room is the key. Does it sound good with real instruments. Deader, pop/rock rooms can be very difficult for them to play in. Also, check they’re not studio virgins. Headphones can really make newcomers struggle. Usually pitching.

kmetal  & paulears , thank you SO much for the insight!!  In my head I pictured the players all together in the studio as an ensemble, so your advice confirms my assumptions that that is the best way to do it.  When the first Cello player I approached said he would record his part at home and send a file it went against my intuition.  He said he did that a few times during Covid, but I would like to avoid it.

As for the space, I have not been in the studio in which we will record yet, so hopefully it is appropriate.  I will do my best to arrange the whole situation where we record all musicians together as an ensemble.  I will also try to pick musicians who have at least a little studio experience.  Great point!

We do not have sheet music so I am exploring the best way to do that.  I have fallen behind on technology but I know Midi files might be of some help.  I had software years ago to convert playing into sheet music but I did not like it.  I may need to revisit that.   Honestly, from what my daughter recorded in garage band the cello part for one song was simple enough to maybe notate by hand.  With notes on where to come in on each part maybe?

I am sure I will have more questions but I immensely appreciate the kind advice and wisdom!!  Truly.

kmetal, thank you for the input

paulears, thank you so much for your very helpful input,

JackHL wrote: kmetal, thank you for the input

Cheers man!

Hi,

My two cents worth;

1. Ensure you are totally prepped. Charts, timing, physical arrangement/setup, mic positions, stands, gobo's within the fore mentioned good recording site (sound wise). Plan with your experienced engineer ahead of time.

2. Ensure the musicians have their parts ahead of time so there are no excuses and they arrived in the correct frame of mind.

3. Be ready to change out musicians if they aren't doing their part. There is no need for one person to cost you time and money.

Cheers,

Tom eh