What's the difference between the 5150 I, II's etc? the II has an extra channel and seems to have a slightly smoother, refined sound, whereas the I is more aggressive sounding.
Should I be looking for a specific year/type? nah, they've been pretty consistent throughout the years.
What should I consider when shopping for them? the only thing i'd consider is price.
Is the 6505 any different from its counterpart? i've AB'd both, they are identical to my ears
Will the combo need a speaker UG? i like the Sheffeild speakers, so i'd say no. personal preference, you'll have to decide that one
i do have to question the weight issue you have with a half stack. the 5150 combo is one heavy mutha. i'd take a head/ cab setup anyday, just for the fact i can split up the weight
. a 4x12 or 2x12 cab or head on it's own is alot lighter than the 5150/6505 2x12 combo.