As indicated before, and backed up by Solitare any of the current Mac line can do the tasks you have identified. Realize also, that any of the new Macs are much better then half decent especially compared to budget offering in the windows world.
With a limited budget, I would upgrade ram 1st to at least 2GB, then look to get a larger hard drive. Look at the 1.83 iMac with 160gb hard drive to start should be enough for a while, or add an external 160HD to the mini for about $150.
Lastly, as Solitare said hooking up to a Windows network/server is not a problem, and most files transfer between systems without problems.