Alright, I have built many trailers from scratch, and modified many others. The trailer from harbor is less than 6 feet long. You would have to ad a completely new tongue, but, since it is a bolt together trailer, the only tool you would need would be a cordless drill. Most steelyard will cut it to the exact length you want (especially if you only buy a partial stick. You buy 14 feet, I guarantee they will cut it to 14 feet.
:lol: )
Now, I agree with the long bunks. On a flat bottom they are king. Also, an extra long tongue is a must. Backs up easier, launches better, can open tailgates etc. But, the low winch post isn't. You make the tongue long enough, and you can leave the tailgate open with a tall post. Plus, the visibility of a tall post makes it easier to load. Now, a winch is a must. For 20 bucks, it isn't worth skimping. I had a damaged winch on a Shorelander I had last year, and loading 3 times with it was a pain. Even if you can motor up to the post, you just hook, turn 3 times, and drive up. Beats the heck out of fiddling with a strap that isn't connected to anything.
I don't believe in anything smaller than 12 inch wheels. Never before burned up a bearing, nor do I plan on it. Bearing Buddies are a must in my book.
You ask how to keep the inside of the steel from rusting, but short of drilling every hole you need, and getting it cut to size, then hot dip galvanizing it, or dipping the whole thing in paint, there isn't much you can do. Just make sure the back and front remain open, so water can drain, and air can circulate.
My only concern with the idea of that harbor freight trailer is the fact that it only has a 4 foot bed, and you want at least 8 foot bunks. The wood needs to be supported a minimum of about a foot from the end, so you would have to ad another crossbar farther forward, of steel. Make sure the bunks extend past the transom, or your boat will obtain a hook in the hull.