Great work on those BattleMechs,
WT!
All the C-in-C models are individual jewels...the problem is, they are micro-armor scaled and therefore very small next to almost every BattleTech model out there. A Scorpion Light Tank would make a decent-sized (and overly broad) SPG, for instance. I'll see if I can find some pictures to give some relative size. The new SF line are a bit bigger, but the "civvie" vehicels there are small as well.
The GHQ technicals are quite nice--as all GHQ kit is--but is rather undersized for you needs. Not to mention overpriced, but they are by no means the only company in this regard. Keep in mind the technicals are roughly 1x2 cm wide/long. If you wanted to go the "armed civvie vehicle" route, may I suggest the following from Brigade Models:
The Springbok jeep (not armed, but not hard to do either, for you, WT)
The Javelot; comes in multiple flavors including ATGM, autocannon, dual-MG, and FISTV/FO versions; a 6x6 pick-up truck that might do the trick as a ~5-tonne light truck
Those two worthies are equally small in comparison to the GHQ technicals, but are more reasonably priced (0.50 GBP, including the ~17% VAT).