As you might already know, CLA means Clean - Lube - Adjust. But, what some repair people call CLA range from tear everything apart and re-assemble to blow dust away and put some oil in it. My local guy charges something like starting fee of $175 for this type of service and it goes up from there depending on what he finds.
Obviously, if you are buying old cameras, the cost of proper CLA will exceed your purchase price and in some cases by a large margin. If parts aren't available, which is often the case, it might be fabricated or somehow obtained from junker cameras.
Unless you want to go very seriously into this type of thing, my suggestion would be to buy one that functions well in the first place. If you are patient and watch classified section right here in APUG, you'll find something like Rolleicord fairly often in functioning condition. I bought and sold mine here too for something like $150ish. Various other makes and models often show up and as being an enthusiast site, description on what works and what doesn't are usually pretty accurate - based on my personal experience.
Also, as mentioned, KEH is a good source for fairly recent models. Their cameras are NOT CLA'd but they do guarantee they work and when they don't, they'll gladly refund or exchange your purchases.
I guess what I'm suggesting is to decide how far you'll want to get into this before you start.