You say about a non-clean system, would it not be sufficient to give it a good clean before moving lenses?
Sounds like you use the viewfinder, remember there is an optional angle finder if you want a bigger view, although coverage of course would be unchanged. Don't suppose you're a fan of live view? If you get for example 600D the tilt screen there could help if you're working in tricky positions.
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.