Go for it man.
There should be two bulkheads in the bottom of the tank in the internal overflow box (the thing that makes the tank reef-ready.). The bulkheads create a seal around the holes in the glass that can direct water to and from the sump. The overlow part - the pipe that drains, is usually supplied by the "mega overflow kit" that you buy before you run your tank. Of course, there are many ways to DIY it if you wish, but it sounds like you pretty much want to plug and play. Measure out the inside of your new stand and find out what the dimensions are and then buy a sump to fit as much as those dimensions as possible. So, for your 90g, it is 18.5 inches deep by 48 inches long. The inside of your stand is most likely going to be 16" deep by 44.5" wide. (Yes, I have been doing this for years.) So, you will more then likely be able to fit a 16" by 30" sump under your stand. Then, look for a sump online that is "rated" for a 90g tank with those dimensions. Pre-made sumps usually come with a little pipe that makes a 90degree angle on one side. Connect the overflow pipe to that elbow pipe on top of the sump... HALF WAY THERE!
So, now, you have your sump and you have your overflow connected to it. A 90g tank needs a return pump that is at least 950 gallons per hour at zero foot of head. Basically,.... buy a Mag Drive 9.5. A mag drive 9.5 has 3/4" screw threads. What this means is - you need to take the pump up to your local hardware store and find some help there at the store and tell them that you need something that can screw down on it. Here's a basic plumbing list for your tank that requires no PVC solvent and cement:
1x hose adapter (they will help you with sizes
1x plastic threaded ball valve
2x hose barb adapters for ball valve
1x length of vinyl hose that will fit on the hose barb adapters and tightly seal. (use boiling water to soften up the vinyl tubing if necessary)
1x adapter that fits your return bulkhead. (If it's a standard reef ready tank, this will be a 3/4" bulkhead. Look to make sure the inside of the bulkhead has threads that you can screw the adapter into.
4x Hose clamps - they are little metal rings that you can tighten down with a screwdriver. They fit around the outside of the vinyl tubing that is pushed up onto the hose barbs of the adapter.
Put everything together and tighten it down. Make sure your bulkheads are really hand tight but not so tight they will break.
Before you fill the tank up with water -
Drill a small 1/8" hole on the side of the black flexible locline (part of the return plumbing) Possibly two holes - on the sides, pointing downwards so as to not create a splash. What the holes do is... when you turn your return pump off, the return plumbing automatically starts siphoning backwards. By drilling those two small holes, you are creating a siphon break which stops the water from siphoning. It keeps your sump from filling up with too much water. Another option would be to fill your tank up and position that locline to blow across the surface of the water instead of down into the water.
As long as you get a sump rated for your sized tank that has the features you want and as long as you do the plumbing where there is no leaks - or resolve the leaks when they happen - and you drill that siphon break - you shouldn't have any issues at all.
