OK - there is a "standard" size for washing machines, tumble driers, dishwashers etc, so the same could well apply to single ovens. But as long as you've checked that the size of the one you want matches the hole that you have got then it doesn't matter what size any other ones are.
From an electrical POV, wiring in an oven is scarcely harder than wiring a plug - you have a 3-core flex to attach, but if you're wiring it into an outlet plate rather than having a plug on the end just remember to switch off the circuit the cooker is on, not just the switch on the plate.
Heatproofing isn't something you
do, you either need to use heat resistant cable or you don't. You
probably don't, but the installation manual will tell you if you do.
I can't advise on any of the physical/mechanical aspects of installation - you may find some useful tips
here
Personally I wouldn't pay Currys £120; even though I've never done one, I have enough general DIY and specific electrical experience to be confident I could do it myself (with a mate to help with the lifting). But I don't know what your skill level is, or how confident you are. If you are in doubt, particularly over the electrical aspects, then maybe you should pay them.