if you really want one as daily driver you're best bet would be a 68 or 69/70 car. they are more plentiful with disc brakes, ac, power steering, etc. than the earlier cars. you definitely want one that has AC, power steering, power disc brakes and preferably automatic trans. a six would be a good option for mileage but you won't find one that has all the power goodies, most were manual steering, manual brake, non ac cars. 70 models have the slight advantage of having locking steering column and shifter interlock as well.
so going by the above recommendations the best car for you to use as a DD would be a 70 mustang convertible with 302 2v (they didn't offer a 4v 302 in 69/70), automatic, power steering, power disc brakes and ac. you'll also want a power top as well. from there i would upgrade it with an AOD (Automatic OverDrive) trans, better rear end gears in the 3.25-3.50 range, add a 4 barrel carb and intake, dual exhaust and possibly a set of shorty headers. wouldn't hurt to also think about upgrading the AC system and converting to R-134A. if you do upgrade the ac for 134 i'd highly recommend upgrading to the sanden ac compressor, better evaperator and ac condensor as well.
the stock disc brakes on the 70 are pretty good as is but it wouldn't hurt to use a semi metallic pad and slotted rotors though. another benefit of going with a 70 model would be that they all had highback seats which are not only safer but also much comfortable than the older low back seats, especially the ones with no head rests like the 65-68 models.
the only downfall to the 70 model was that they made a lot less convertibles in 70 they did in other years, but they aren't all that hard to find though.
here is a perfect candidate for what i'm talking about
that was just a quick ebay search. this car doesn't have AC but it has all the other stuff i recommended and ac could always be added later if you wanted.