When you say six brands have caught your eye 5 and 6 I presume is one brand i.e Humber? From your indented use I would say that rules out an Avon Searider and the Avon Adventurer isn't a great boat in the water.
Which leaves it to Solent, Ribcraft or Osprey which are boats with superior quality to the Humber. ( humbers are cool boats though)
Ospreys are superb sea boats
Solents are very nippy and well made boats
Ribcrafts are a good mix between the two
Engine size I suspect you'd find the perfomance of a 115 4 stroke limited after a while so I'd at leat look for a 140 4 stroke, but then I'd prefer a 150 Opti for that size boat
If you are looking at the used market with 20 k in your pocket Don't discount the Ribtec or XS 6.5 If new then Parker are well worth a look If you could go to 7.2 then the Shakespear 720 is a cracker and could fit in your budget.
When buying used only allow for 25% of the electronics value in the price you pay, As it's likely to have been superceeded and is out of warranty
Also The Solent is a splash of the Tohatsu one design which was a very well made boat. Laminating by Porters and toobs by Henshaws , so it may well be worth piclking up a good Tohatsu and slapping a new lump on it
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo