I would agree that 46 x 18 is a good ratio for a guy like you to start with. 46 x 20 is probably a bit too low, even for an old guy on moderate hills not trying to go really fast.
Looks like you're starting out with a pretty good platform.
If you're unwilling to drill the fork, then yes, a clamp-on brake is your only option. I mean, there's a constant debate about brakeless fixed gears, but you seem like a reasonable, mature, safety-minded individual, so I doubt you want to go there.
Deep drop (pista) handlebars are good for looking cool and for track racing, but not for comfort or for mounting brakes--pretty much limited to cross levers. There are tons of good riser bars available to you.