Dp,
I landed at Reus two years ago so I'm afraid I can't remember what the landing fees amounted to. There certainly was no requirement for a handling agent at that time.
If you clear customs and immigration in France then you do not have to do so again in Spain (Schengen). There is nothing to stop the authorities from carrying out spot checks, of course, as happened to me in France on my way back from Spain last year.
As regards flying in Spain it's pretty straight-forward. The trip into Reus was part of a trip down to Africa and on both the southbound and northbound legs the service from ATC and airports was second to none.
BTW I visited the new resturant at Weston last weekend. Great spot. How long has it been open?