I've been looking into a modeller, but I just want the one sound, and maybe be able to add some delay.
Can it do medium gain Marshall type stuff, with the dynamics of heavy picking getting you into heavy territory, and a roll back of volume and soft picking semi clean?
Hunter's gonna be your man for a definitive answer, but I would suspect YES (if Line6 and VOX can do it!)
I've been using Line6 PODXT for a couple of years and recently converted to VOX Tonelab LE.
The PODXT does what you ask, but with a processed sort of sound. It sounds like the guitar on a record, rather than the amp in the room with you.
I got the Tonelab because I think the sounds are better - more like the amp in the room - and I didn't want to go into AFX territory budgetwise (not that I checked how much they are!). The Tonelab does what you ask, and better than the PODXT in my opinion. Although I really switched for the VOX and Fender models (much nicer in the Tonelab), it's the Marshall models I use most, and I use them exactly how you describe.