1. I know for a fact that, even in a web based environement, Access could handle without fear about 50,000 lines of records (around 54 MB of data).
2. If you fear that they would have problems in managing Access, I'm afraid MySQL will... hm, destroy them
! That's the main problem with open source applications (if you deal with Linux, you'll know what I mean
).
3. my advice is to ask someone to design a web based application, put in in a secure environement, with a special designed log in system (that is people with a certain credential should be allowed only to certain pages) and your clients and agents would then just log in their respective areas of interest and check/update/search for information they need. This way you'll deal with a simple and customized application that for sure will match your exact needs. This is not very complicated and should not be very costly. I'm sure you can find even on this site someone to do it for you (me included
).