OK to answer your questions:
1) most ROMs out now are for Gingerbread (2.3.x) which all require a newer radio. i have had great experience with the 802 radio but anything after MR2 will work for you.
the Radio is flashed as a "PG05IMG.zip" file through HTC's Hboot.
2) then you have to decide between AOSP and Sense. they are both stable and reliable, it is just a matter of what you want. if you like sense, stick with sense. if you like AOSP, stick with CM7.
AOSP roms have their issues like GPS not working, and general location weirdness, but there are workarounds to get these to work.
sense ROMs are usually a little slower and less customizable, but they work and look great.
for AOSP i would suggest CM7 or OMFGB. tons of options, very customizable, and really fast.
for Sense, the general consensus on the forums is that bamf is the way to go (i am currently running Bamf forever 1.0.3 and it is amazing) but i have had good results with skyraider and liquids sense ROMs as well. just try them out, and see which one works best for you.
a custom ROm will get rid of the "bloat" there are other things you can remove as well. i would stick to apps only though, nothing in the system, or processes or anythign like that.
i have frozen stock: music, stocks,news,messaging, email,gmail,camera, fm tuner,hotspot.
one of the tips i can give you, download titanium backup and the Pro key. it is kinda expensive, but if you plan on switching ROMs a lot, it is a HUGE time saver. at the very least get titanium. open it up, menu>batch> and do a batch backup of all user apps (DO NOT BACKUP/RESTORE SYSTEM APPS EVER!!!!)
when flashing a new rom a Data WIPE is almost always necesary, so this comes in handy after the flash.