Macs and PCs

I've been asked to design a database for up to 60 concurrent users. The
awkward side of it is that the company in question use a mixture of Macs
(OSX) and PCs (XP) attached to an OSX Mac network. There will be around
50,000 records altogether.

I'd like to make an Access database so one solution would be some sort of PC
emulation software on the Macs. Alternatively, perhaps I could get
FileMakerPro on the Macs to read an Access back-end database (or some other
database format that both packages can read).

If anyone out there has been down this road before, I would very much
welcome any ideas. Also, will 50,000 records make a typical Access database
too slow for comfort?

Thanks.