like how much fuel it uses per 100km?

pls refer to the main EP3 thread, we were just discussing this very thing.

how reliable will it be in the long run?

depends on how you use your car. but it's a Honda so it should be fine if you treat it well.

how hard is it to buy engine mods and cosmetic mods etc?

with engine mod, it's just a K20 afterall. with cosmetics its hard as not many option available for body kit etc but you can always do wheels and Mugen stuff.

what the experience was like owning one of these cars?

if you like your vtec, it's great. if u like turbo, buy something else.

also i have found some with only 40,000km+ on the clock. how trust worthy is this number as it seem awfully low for an eight year old car.

you should always buy with condition.

and also will it be a perfect car for a P plater.

its not too hard to drive so a P plater should be fine but whether it's perfect for you, only you can answer this question.