I think your been a bit hard on yourself to find the perfect boat, my experience of buying and selling are exactly what you have experienced.
I'm afraid to say you will not find a perfect boat with everything you want for the money you have, i recently tried to sell our precious wellcrafty for 9500 quid(sorry pound sign not working on k/b) and we had no takers only people who wanted to give 5000 grand, No way would i sell it for that, the snipe trailer along cost me 2500. the original trailer was an American eagle trailer and yes illegal over here
While selling wellcrafty we looked at spending 15000 on a bigger sports cruiser 24/25 ft max, everything we looked below this figure was either a dog or not looked after, stuff missing nothing was perfect, so we looked at other boats around 18-19000 quid and there were beauties everywhere, but just out of our cost and none had trailers which added another 3-4000 quid on top, so we settled back with our original boat wellcrafty and that was the best move i made currently. and we are slowly adding extras to it now to make it more our boaty thing.
The honest part im trying to say is many boat owners love there boats i brought mine in a really naf not running condition for 3575 quid inc eagle trailer, what i have done in time and money its worth so much more to me and i will neve rlet it sell for the going rate of this boat, this is what you are finding from owners that are selling ther boats are worth so much more to them, as your boat will be when you buy one and get it to how you like it
The comments you have had about getting a boat with this replaced and this serviced is in my opinion silly, do you buy a second hand car and ask the seller has the radiator-clutch-brakes-fuel pump have been replaced has it had every single service from it start of life at the dealer, unless its from a dealer or very over priced then i would assume the answer would be no.
The Mariah sound a good starter boat and don't let it slip away because of a trailer issue, i could of done this and i would not have the boat i love now, a quick search online for second hand trailers will show there are lots of them, also look at ex-demo witch is what our snipe twin axle was.
Well my kids now 4 and 5 have loved every trip out, but now no way every possible pending trip we have had recently they don't want to go, i agree trying to get you kids into it but things will change and they may hate it later.
There are some very lovely honest lads and lasses in here that are lucky to own a lush floating cruiser, to them spending extra to get what you want is the best thing, but to us trailer boater who do everything on a shoe string adding extra is not possible, if it was me i would get the mariah get it home or boat yard while saving and looking for a trailer work on the boat and engine for this year get every done and ready for next familiarize you self with the engine, do the servicing your self inc the leg service , you could then look back and have a cracking nice boat and trailer to fit your every need and you are armed with the knowledge if anything goes wrong you know what to do.
This is only my opinion and not to offend the usurers of this forum