WinME does limit you on the more complicated themes in that it doesn't support transparency. Many themes now use transparency for some eye-candy so that may be a problem as most themers don't think of 9x machines being in existance. Personally I think WinME is crap anyway, hence it being part of Windows CEMENT. SOME machines that came with WinME can be upgraded to XP, but you will probably be better off with finding a copy of 2K that is laying around somewhere.

The not being connected to the net thing will be a bigger problem until you find a theme you want to use. OTS2 themes depend on the Internet connection being there as most of the people of the world have broadband. I'm not saying it is a requirement to be constantly on the net, but you will need to get there from time to time for module downloads and the like.