I can see why you would want to play around with a Mac before getting one. The concerns you have would take a good bit of investigation to find whether or not it would work.
If those rental places end up being too expensive, there are some alternatives that could be cheaper:
1) You could buy the powerbook that you think would suite you, test it out and if it works great, if not ebay it off at a slight loss.
2) Check out the Marketplace forum and maybe post to see if anyone lives in your area and might be willing to part with their powerbook for a while for some compensation.
3) Beg someone you know with a mac to let you test out your inquiries
The .net stuff might be a show stopper, even if you succeed to do the things you want it to do in VirtualPC it will be incredibly slow.
http://www.itwriting.com/dotnetmac.php
As for cell phones, I know there are plenty of users that have successfully gotten their cellphones to connect to the internet, mostly through Bluetooth. However if your using a usb connection, a driver might not exist for the Mac side and if it’s a serial connection, it definitely won’t connect. I recommend checking out Howard Forums for you cellphone model and seeing if anyone has tried to connect to the internet using a Mac:
www.howardforums.com/
The Palm will definitely connect up no problem with isync…the applets will definitely require some snooping around the net. They may be shareware to do something similar to what your current apps do. I’m almost certain you will have to find a work around for at least one, if not most of your applets. Don’t think Microsoft Money even exists on OS X, but there might be someone out there that has written a program for it, wouldn’t be surprised.
Best bet is to do your research on all your areas of concern before dropping money on something you can’t keep or wasting countless hours trying to connect your phone up when others have tried and failed. Where there’s a will there’s a way – it’s just a matter of how much time and money you want to waste.