I have a bunch of T5735s also but for other uses, you can without problems replace the Sempron 2100+ (1GHz S1G1) CPU with a Sempron 3700+ (2GHz) without heat issues, I have done that in a few of them, also soldered another so-dim slot on the empty space for one for a extra ram stick, tho not really worth the work since you need to source double height slots since it will overlap the other.
If you want a bigger CPU then the 3700+ you have to update the microcode for the BIOS, easiest way is to just burn a new EPROM.
Also there is no problems at all to install Windows XP on the machine (tho using nlite you can get a really small footprint windows xp), Drivers are easy enough to find, just read what chips it got, the nic can be hard to read tho but if needed I got the correct drivers.
Also I use CF, DOM's or real 2,5" HDD's in my systems tho 2,5" HDD's requires you to make a hole for the flat cable and mount it on the outside.
I bought a big bunch of em for less then 30$ each.
Good luck selling, the price feels a bit on the steep end for what you get, it's a cute little machine but the 1GHz low power CPU in the system is a bit on the slow end, upgrade to the 1.8GHz or 2GHz CPU does a lot and cost of em are not bad at all.
