Firstly, I have to believe this is some sort of troll given that someone wanting to buy a light a/c must have the capability to use a search engine, review the countless threads here, or just talk to their local flying club....but OK, I'll bite...
Caveat..Others will have different figures/reasoning. This is what it cost me.
What does one need to do to buy an aircraft? Let say for eg. a C172 or P28 Warrior 4 seater?
Have cash, go look, use friendly engineer to check a/c and docs. If you like it, pay money, tell CAA.
If one wants to personalise the registration, how much does it cost?
It used to be £250 but don't know now. Ask CAA or check website.
Whatis the insurance cost?
Depending on what you want to use it for, abour £2000
Regarding its home, do you think it is a good idea to park it at a flightschool and perhaps loan it to the school for training and in return get free parking, maintenance and monthly rent for the plane?
You will not get "free" parking or maintenance. If you buy an a/c then you buy it for the ownership of something that is yours, available when you want it. Otherwise, why not join a group?
If one owns a C172 for example, what do you reckon would be his operating cost per hour in terms of fuel?
The fuel will cost the same whether you own the plane or not. If you mean what are the running costs per hour of your aeroplane, it depends how often you fly. 1 hour a year will cost a lot, 100 hours less so
What expenses must he budget for monthly/annually etc?
As above, my experience on a TB9 was you won't get much change out of £800 a month..hangarage, insurance, maintenance etc
Can you please recommend a used aircraft for sale website as well as any website for new aircraft owners??
Google "used light aircraft for sale" and take your pick