to use with alternative firmware's are the wrt54 gs series CGN0 - CGN5 still the best to buy, 2nd hand that is? Or are there newer more powerful wrt's that are able to run f.i. Tomato?
There is a WRT54GS (v.2.0 & v.2.1) with 32/8 MB but with 216 Mhz CPU (CGN0 has only 200 Mhz). Table on wikipedia.
Our own Autopsy: Linksys .WRT54G and .WRT54GS Hardware Versions Under the Knife will show you. we have not updated this to add in the v6/7/8 however the WRT54GL is just as good tbh and it comes with a warranty if bought new.