are you going to be alright on the (~35mm?) width tyres that will fit in that frame?
Re the hub gear if you use an old (non-NIG) 3s AW hub I would suggest that you carry a spare axle and a few other spare parts such as pawls and pawl springs, toggle chain, toggle key etc. You will need cone spanners, a drift (screwdriver bit?) and a hammer (rock?) to gain access to the internals if needs be. If using a (secondhand) steel shell the wheel should be built using the original spoking pattern in the hub (the sharp edges on the drillings will be slightly rounded) and also use spoke washers.
To get low gears without a lot of extra complexity there is a cunning dodge which I have used which I call
'Magic Alpine Double'.
The idea is that you have two chainrings and two sprockets (which an old AW will accommodate quite nicely) and you use the inner ring with the inner sprocket when climbing hills and the outer ring with the outer sprocket the rest of the time. There are two things that make this easy to live with
1) that you run the chain slack enough that you can simply move it to the other combination (using a twig or something so you don't get oily fingers) without moving the wheel. If you get it just right the chain won't fall off when you are riding the bike normally, even if it is pretty slack and
2) that you use chainring/sprocket sizes which give an exactly identical chain length. (You can verify this in a chain length calculator)
For example I have used 46/19 and 36/30 combinations this way using a five speed SA hub
The gear ratios that you get this way are represented here (for both S5 and AW 3s hubs)
http://ritzelrechner.de/?GR=SAS5&KB=46&RZ=19,39&UF=2170&TF=90&SL=2.6&UN=MPH&DV=gearInches&GR2=SAAW&KB2=46&RZ2=39,19&UF2=2170(note that to represent the 36/30 combination I have used a 'proxy sprocket of 39T; 46/39 is almost identical ratio to 36/30; the gear calculator otherwise gives you 46/30 and 36/19 gears which you won't be using, if you enter the chainrings and sprockets 'as is'. )
In both cases you get two ranges of gears; one of which is useful on the flat and one of which is useful for longer/steeper hills. Obviously you need to dismount to change ranges, but other than that it is a very simple (and almost idiot proof) method of obtaining a good gear range on a bike which is only a little more complex than a singlespeed. In both cases gear range (total) is from the low twenties to about 90" or so.
I modified a 30T cassette sprocket to fit onto a standard 3-lug driver; standard IGH sprockets don't come bigger than about 26T or so IIRC.
cheers