this is just my personal opinion.
I have had I think 5 Fiberglass boats and almost that in Alum boats also.
one thing I do is ill do my own upgrades like Electronics Power poles and so on myself. with the cost of this stuff these days a decent pair of Electronics is gonna cost you $3k. now you throw that into the financing and your paying X-amount of interest on that $3K worth of electronics for however long your loan might be for. me ill suck it up come up with the money for them and do the install myself where I'm not paying interest on them.
as far as your size of boat. that is all depending on the types of bodies of water you fish. here I wouldn't think of getting into anything smaller than 19'-20' but we have some big lakes and some can get really rough at times to the point even a 20' boat it gets a little sketchy. have been in some really nasty stuff over the years and thankfully no problems but I wouldn't have wanted to be in them in an 18' boat either.
a boat is huge investment but its all based on your goals and what you want to get out of it.
if I was fishing mainly smaller bodies of water that didn't get really rough I would have no problem with a smaller boat but for me its not the case.
the boat I have I just bought and thought I got it for a good deal and still do but I'm still doing things to it that I want. its all about what you want out of it and what your willing to put in it.
again just my 2 cents