What does it include with the engine?, is it literally the engine alone or does it include batteries, exhaust, stern gear, and prop?. Remember when it comes to fitting and engine there are difficulties. If you have a company fit the engine it means you will get warranty from the supplier, if you decide to fit an engine yourself I do suggest that you read the manufactures instructions carefully. If you were to fit the engine but you don't follow all the instructions that they set out you will find that if something is wrong with the engine you may find getting replacements on warranty difficult.
If fitting yourself the alignment is quite important depending on the coupling that you fit, things like python and aqua drives can make alignment less difficult but once again read what there requirements are before fitting.