Mike,
Where do you live and what is your AC usage? You want to select a SEER rating that makes sense based upon how much your AC will be run. After determining the SEER rating that you want, be certain that you are comparing the same SEER rating when you are comparing quotes from different installers. Remember that a low SEER rating is adequate if your AC usage is low or even if it is moderate.
If you are attempting to save money on installation, then go with an R22 compressor properly matched to your existing system. The R22 is still inexpensive and works pretty well. It is single component refrigerant, which is a big advantage if you develop a small system leak. The R410a is a "near-azeotropic" blend of two refrigerants. It has low but significant fractionalization (which is why it must be charged as a liquid) and any leaks will cause a shift in the component mix. In plain English, even small losses shouldn't be topped off; the system really should be evacuated, fixed and recharged whenever any leak is detected. With R22, very small leaks can be legally and practically topped off (with some restrictions).
Do not attempt to tack an R410a compressor to your existing system. Avoid any installer who would do it. The operating pressures for R410a are too high for your existing system. To properly retrofit your AC system for the R410a compressor, you'll spend as much as you would to install a mid-range new system.
Personally, I'd recommend a new system that fits your budget. Would you spend $400 patching together a 20 year-old refrigerator when you could buy a new one for $600? Likewise, it usually doesn't make sense patching together an old home AC system. I'll be attempting to patch together my next door neighbors old R22 system this weekend, but we are purchasing materials at wholesale and our labor is free.
In your case, going with the new system will give you (1) a system in which all of the components are properly matched, (2) a new system warranty, (3) a lower operating cost and (4) a system which should need little maintenance for quite a while.
No matter what you decide, get many quotes and opinions.
Question for you: Are you 100% positive that you need a new compressor? Sometimes the need for a new compressor is very obvious, but sometimes it isn't. Be particularly careful of the situation in which one replacable component outdoors is bad, but you get pushed toward replacing the entire outdoor unit. Next, you are considering buying a complete new system. I've seen this happen when the only problem is a compressor fan motor which is tripping the breaker. Sometimes a bit of oil is all that is needed; other times there is a need for an inexpensive start capacitor or run capacitor; or sometimes a homeowner-installed replacement motor will fix the problem (this is certainly extremely inexpensive compared to a new compressor unit or a new system).
Good luck, Gideon