#1
Just found a Marshall JTM-60 combo for $645.

It's got 3x10's speakers, and looks pretty old. Thinking about trading my JCM900 head for this.

High as tits
#3
Don't. It sounds fairly fizzy when overdriven. It also is notorious for its overheating problems. I owned one for a week and returned it because the circuit board had been burnt from overheating.
Guitar
Ernie Ball Music Man Silhouette Special
Washburn WI64
Martin D-15
Amps
1960s Harmony
Rocktron/Egnater TOL 50 2x12
Rivera Chubster 55
FX
Fulltone OCD v.1
Keeley TS808
Voodoo Labs Superfuzz
Wilson WH10
Boss DD-20
#5
If you're prepared to spend a few quid fitting an external fan it's alright but I'd personally pass on that deal. Marshalls don't sound too great with ten-inchers either, totally the wrong type of speaker for the tones they typically produce. If you're currently a JCM900 player you may find the cleans a little better, but the overdrive a bit on the raspy, farty side.