Sound man. Fill and pressure gauge are off the shelf jobbies, not cheap though. Had a Mk1 myself and found the trigger a bit 'Less sensitive' but its generally hard to find a Mk1 with a as new trigger unit. They were good back in the day. Not sure you can drop in a Mk2 unit or even better the MFR unit. Same trigger thats on my HM.
Keep us informed about this Mk1 Be nice to know how it turns out.
Im after one myself to be honest, 177 probably a Mk2 really, but just waiting for that right example, with in fact a fill and gauge adaptor. Need to flog my 100 first though.