I might keep looking. I adore Welsh ponies, and they tend to dominate the pony hunters in my area and make great sport ponies, but this little guy doesn't look like a very promising prospect.
Please bear in mind that I basing this on one casual and unflattering photo, but what I see is an upright shoulder and a very short stride, not moving out from his shoulder at all and not even close to tracking up. He's also built quite downhill, and has a neck and shoulder connection that's going to make it very hard for him to travel in a good hunter frame or a correct dressage frame. So, for your $700. Investment, and your time, you're going to end up with a backyard or pleasure pony worth $1500., maybe, as the market for medium ponies is *very* limited, green medium ponies even more so.
If I were looking for a prospect to flip; first, I'd look for something with better movement or conformation that could finish up as a show or performance prospect. I also be predisposed towards a large pony, as the market is much larger.