When you say it's a bitch, How exactly does it react Ellie?

Could potentially be a number of things. Faulty coolant temp sensor can effect cold starts, when they fail they usually make the ecu think the car is hot and it won't call for the cold start routine.

Compression problems (worse case scenario) When they're losing compression from something like burnt exhaust valves not seating the symptoms usually extend to once it's actually started the car will be a bugger to drive until it warms up, ie pulling away from junctions etc the car will hold back and bog down. Also the engine will generally run a bit shakey which will be even worse when cold.

Battery. Even though it might seem ok, if it's not really good and not turning fast enough it'll cause probs, again exagerated by the cold weather. Know anybody that can perform a drop test on the battery?

It's also worth not trying to start it too fast. Turn the key to the ign on position so you hear the fuel pump prime and stop, then maybe turn off and onto ign on again and hear it prime. Then try starting.

Do you know what plugs they used? Was it someone that knows imprezas? Should be NGK PFR6B's

Personally if it was mine and the battery wasn't turning her over really fast, I'd change that. Then you could try a coolant sensor if that doesn't sort it.

It's worth changing the MAF on these cars anyway, the original 99/00 ones were pretty fragile.