for that purpose, maemo as is should be sufficient. there are repositories both from nokia and maemo and both stable, testing and devel.

so, everything for learning should be available, including clobbering your installation.

you can apply your ubuntu knowldge, plus: if you fubar your installation, you can always reflash and start from scratch (easier than reinstalling ubuntu, since the flash images are preconfigured for the n900, no need to install extra drivers and stuff).

as the market goes, there are only two phones available with full linux: the now discontinued, though still sold, openmoko freerunner and the n900.
the freerunner is available at a range between 200 and 300 euros and more open than the n900 with several distributions available, among them plain debian. as a second device or for a geek not minding killing his phone now and then, it's an option.
if you need a full working phone most of the time (even the n900 lets you down now and then) and consider full linux access as nice to have, go with the n900.
according to some "that's it. i sell!" posts, there should be some 2nd hand n900s available ...