Dunlop makes strap locks which have a bit that you attach to your present strap and another bit that fits into the hole where your current strap buttons are. They're pretty cheap, probably no more than $20. If you run a search on "strap locks", I remember a topic on this a while back.

I think that 100W is way too much power for your current needs. You probably would never turn it up past 3 or 4. My 45W amp is just way too loud, even at 5 or 6, and is literally ear-drum shattering, without ear plugs, at 10.

If you're still considering anything over 50W, just ask Lordathastrings for his opinion...