Hi Ollie,
I'll give you my two pennyworth. The AFN is a good engine indeed as monsho says, but the bhp of tdi engines depends on many things. The 1z and AHU are virtually identical, with the only difference being in the turbo they used (AHU a little more max boost I think), and I think it had an 11mm fuel pump, and the 1z a 10mm.
The AHU and 1z fit well in an engine (look at my garage picks of the engine). Russel at Syncronutz sorted mine out as it was initially a bit of a mess - he is a total guru at this stuff. You need an intercooler, and you need to make sure the ECU is in the dry if you going for "fly by wire". Russel also builds the Tdis with mechanical pumps which are favoured by a lot of people. I like my electronic set up as I can have cruise control and slightly better fuel economy.
The bhp depends mainly on the turbo, the injector nozzle size (you might need a bigger pump if you go really big), and then a remap. The advantage of the stock 90bhp 1z (with .184 nozzles and the Garrett GT15 turbo), which are quite common, is that you can probably manage with a normal 215mm clutch without burning it out. If you go 110 you will definitely need a modified flywheel to fit the 228mm clutch or it will slip.
I run bigger wheels and a 3H gearbox, but as monsho says you should really think about getting in touch with Aidan for a box that will take the torque and will run at lower revs. I'm happy for the moment going around at 65ish (I can do 80 but it's very loud

), but when I get the cash together I'll get a bigger clutch, stronger and longer gearbox, vnt turbo and bigger injectors. For the moment I'm happy with my setup. Some people say that with a 1z and 0.7 fifth for example it doesn't have quite enough power to pull up hills.
I think I read somewhere that the AHU block fits without having to modify the engine cover, but the AFN might need it modifying to fit the pump. I may well be wrong about this.
Anyway - I'd go for a 1z and then mod it if you want more power. There's hundreds of posts about modding tdis on the web - they are great engines. Even really cheap mods like the switched "evry mod", which is very popular in the states... I couldn't possibly comment
Hope some of that makes a wee bit of sense at least!
Ewen