Firstly, new with your budget will leave you with a pretty limited choice, and probably not much of an upgrade. Here's my two penn'orth,you may get conflicting views.
A fork needs to work to be of any use. There are loads of forks out there as you notice, and loads are for sale because folk love upgrading them. Firstly, let's discount the crap, because, quite frankly, there's loads of crap out there for sale, and loads sold as OEM on new budget bikes. So, coil sprung Suntour with limited rebound damping and stiff coils so you can barely adjust the compression or preload can be struck off your list.
To help you navigate technical trails your fork needs to track the ground by compressing where you want it to absorb impact, and then return to it's starting position (rebound) in a controlled way. If it just springs back to where it was as quickly as it compressed, it would just spit you off the bike, so you need a fork with rebound damping. You also need to be able to adjust the rate at which the spring works to suit your weight and riding style, so that the spring compresses as much as you want it to without plunging through its travel all in one go. Coil sprung forks do this via a preload setting, and/or swapping springs for softer or stiffer versions if the preload is not within your weight range. Air sprung forks do this via increasing or decreasing air pressure inside the spring to suit your weight.
That's all you need. Everything else in terms of bells and whistles is frippery to an extent (although some of the frippery can be fun or useful, but not essential!) so you don't need remote lockout, pushloc, poploc, you don't necessarily need adjustable compression damping, motion control, flight control, dual air, dual position, dual crown, duelling banjos or turbonutterbastard buttons.
The budget you have will get you a good quality, working, useful fork. Coil forks are simple, easy to service (as are all Rockshox by the way, you just need to be brave to do them yourself) and in your budget look for a Rockshox Recon or Sektor fork. You should get pretty near new for those models. Next on my list would be an airsprung fork, lighter than a coil, but not necessarily better, but a saving of about 400g so not to be sniffed at. Here I'd be looking for a Rockshox Reba, dual or single air, or an X-Fusion. Suntour do some decent air forks with their Epicon and Raidon models.
You may find a fox float within budget, but in my experience Fox forks tend to be a bit flexy in XC flavours, and need servicing twice as often as Rockshox. They are also more expensive!
Lastly, if you're buying used look for unmarked stanchions, service history, and check that the steerer length and diameter fits your head tune and headset. Also don't forget to make sure the fork dropout fits your front wheel axle, or that your hub can be adapted to fit the fork's axle/dropout.
Adam
