A G4 (Tower) shouldn't set you back too much. The mini and iBooks are pretty much the slowest systems Apple has available. Though they were given a little speed bump recently and should be more than adequate for mild use. A number of people here have them and as far as I know, are quite happy. I wouldn't consider a mini to do software development on of course - unless you like going out while waiting for compiles. But your typ[ical email and web browsing would be fine.

A number of Macs have been configured from the Apple store with too little default amount of RAM. Make sure any machine you're using has at least 512MB to get a more realistic taste of its usability.

I'll ask this question to the guys on the beta-testing forum I run to see what they suggest. They should also be able to guestimate a an average resale price you should expect to find.

Bruno