I think I would steer clear of the ST170 gearbox too to be honest, as it's going to be a bit more hassle to fit. The standard iB5 isn't the strongest, but should be ok with a fairly standard ST170 engine. It's been a while since I messed with any Zetec Escorts, but you should be able to bolt the ST170 engine to your existing gearbox and use an Escort flywheel and clutch. That way you can retain your standard drive shafts etc, which will make life a lot easier. The only thing I'm not sure about is whether the bottom two engine-to-gearbox bolts (the ones in the sump) will line up with the ST170 engine. There may be a way round that, but someone else might be able to advise. Another option is to use the MTX75 gearbox from an RS2000, but you would also need the hubs, drive shafts, gear linkage, sub frame etc etc. The gearbox is stronger, but I'm not a fan of the standard linkage in the Escort. It's a bit baggy with some wear. You could fit the Focus MTX75, but you would need new mounts etc, so as much work as fitting the ST170 box really.

Mounting the engine itself isn't the worst thing in the world if I recall correctly. There is only one mount at the timing end. I think only two of three bolt holes to the engine block line up, so you have to modify the mount if you want to use all three. Otherwise, it's not too difficult. You can use the ST170 exhaust manifold, but I believe you may be able to use an Escort manifold too (Ashley used to make one, I don't know if they still do). The hardest part is the inlet and management. I'm not sure if it's possible to get the standard inlet in, or if you can run off the Focus ST170 ECU, so you may need to go aftermarket.

Another option, which is simpler, is to fit a 2.0-litre silver top Zetec. They're probably not that common anymore, but you can fit one of those pretty much straight in if you fit the sump, oil pump and water pump from a 1.6/1.8. Use the inlet from the 1.8 115PS engine and you can even fit the 2.0 ECU by swapping a couple of pins around. You could also fit the regular 2.0-litre blacktop Zetec, which would require the above hardware mods in terms of the gearbox/engine mount, but you could probably get away with running that from the silver top ECU. Both these options would be a bit easier from a wiring perspective and give you most of the performance.

Just depends how far you want to go really.