Unfortunatly mate there are very very few engines that can be swapped and run on the same ancilleries. When going for a larger engine you need to allow everything else on the car to keep up with the new extra demand. Such things like your fuel pump and ecu should really be the only external parts needed as injectors, fuel rail. If you go for the bigger engine, even if its onlu 0.1ltr bigger, as a minimum a remap would be needed so the car knows to give the engine its extra fuel etc. Suspension is a biggy as your new engine will be heavier and will either ground out easier or kill the shocks and springs quicker from the extra weight. Then you have to think about stopping the extra power/weight so brakes would be needed aswell.
But as far £££ per BHP, your best bet is actually the 2.0 V6 MVIC FTO engine. They are seriously easy to fit into a Colt or Carisma so why not think about one of those. Pick up a cheap donar car and you can get the brakes, fuel system, ecu, engine, loom everything really and would save you so much in the long run.
But if you wish to go for a cheapest mods and still gain at least something. first things usually on anyones list would be:-
Exhaust system = anything between £150-300 depending on your pocket.
K&N or Pipercross Panel Filter= roughly about £40-60 depending on car.
Remapped= remap rather than chipped as its easier to do again if you add more mods usually costing about £200-300 again depending on car.
That little list will give you a small noticable difference in throttle response, revs and pick up of the engine and should see you about 8-10bhp on a 1.3. The 1.6 gains approx 8bhp when remapped alone.