I would probably wait until you start your course and you can see how a table feels.
I'm not sure where you are in the world but the Massage Table Store is quite good, they have a showroom and you can wander about and try different tables for size, I think they are south of Birmingham somewhere. Gloucester maybe?
I have a wider table, I prefer it, I also have thick sponge on it which I find is good as I do remedial work and no hands massage and it's more comfortable for the client with these types of treatments. If you're going mobile you might want something more lightweight.. I'm mobile and I wish my couch was lighter, fortunately I rarely take the couch out on visits. Back rests - this depends on what you want to do, I don't have one and have never used one so unsure what it would be like, I do reflexology but have another chair for this. Arm rests? Another thing I've never used, but I have a thing on a face cradle which people can slot their arms into if they want, saying this I rarely use the face cradle. I'd say make sure you're ok with the heights and that it's adjustable and have a good lie down on your couch and make sure you'd be comfortable with it. Just don't rush the purchase cause you're stuck with these bits of it for a while.
If you go up to the top of the forum and do a search on massage couch you'll get a whole host of posts which may give you more info.