Buying a RGN need opinion on what to look for.

Most versatile in my opinion is a flat deck. A beam is too much work for me although anyone with a beam trailer has my full respect because it is a lot more work with throwing cribbing around a lot.
I assume you will be using a 4 axle truck. If running 7 axles I would say 24' well would work fine. You could get by with even less deck space then that, but you have to watch your inner bridge when permitting. If it is 8 axles (quad or 3+1) then typically a 26' works well for the fact of using the extra deck space to get your weight distribution correct.
Finding a trailer of that caliber will be very rare to find a tandem with 3rd flip.
Rust...check crossmembers first. If they are peeling off sheets of rust then there maybe a problem. Hit the crossmembers in different places with a hammer to see if it punctures through. They can be replaced, but could be rather costly.

With a $30,000 limit that will probably not allow you to have a lot of choices to pick from. I'd say find something solid and run it a year or two and get your cashflow and credit built up and then upgrade. Be careful if going to auctions. Most trailers at auctions typically are someone's problem they want to pass off to someone else. I'd say stick with private sales or dealers. Both are hard to gauge if they are honest or not. If it is a trailer with fresh paint be careful when looking it over. Liquid overhauls typically are trying to hide something. Not always, but often.

 

Some people call it a raised beam. I call it a dropside deck because the main beams are the same height as a flatdeck...its just the outer beams are shorter getting lower height with machinery excluding excavators. I say that because its not the cab your worried about on height, its the boom. So no benefit there for an excavator.
Hauling small rollers or pavers, small dozers even may be a problem with a dropside, but if you are not hauling those things and find a good deal on a dropside then I'd say have at it. A dropside will tend to be a little heavier then its flatdeck counterpart.

 

It will certainly bring the height of an excavator down, drop your machine 6" you drop your boom heal 6" which drops your overall height.

 

Normally the center beams on a drop side will be taller than a standard flat deck.

 

Technically true, but that typically falls mainly if you are talking about 65ton capacity and up. Looking at 50-55ton flat decks (he brought up 50ton+ 3axle) most center beams are 16" to 17" tall. That is typical height beams of a flat deck. Dropside decks' main beams are the same for the most part. I did come across a rare 55ton Trail King dropside deck recently though with 18" main beams.

 

I pulled a Trailking drop side 55 ton, center rails were at 24" with 80,000 on the deck, the drop sides were at 18"