Computer Choice

Gentlemen:
I am considering getting a newer computer to replace my 133 MHz unit.
I will use it for either a DOS based controller or a MachII which
needs WinXP, I understand. A used unit around 500 MHz might be OK!
What kind of connectors do I need for the parallel port cards? Are
ISA slots in the later models? Will I have any problems in using PCI
cards if I stick to DOS programs? And how much should I expect to
pay? My present unit was $35 two years ago.
I get by with Windows but certainly am no expert!
Thanks,
Jack C.