OP, if you have plenty of options for placement, buy a quality sub and then down the line get a second one. I say because of placement because one can be solid if you place it in the right spot (many times closer to the listening position) by doing the sub crawl or testing multiple points and testing with frequency sweeps.
If you don't have a lot of options for placement, buy two cheaper subs now. That will make it so you can fill out the nodes more evenly. One great sub in a poor location is not as good as two decent subs in poor locations. Just dial in the peaks on each sub if you can. Rhythmik F12 does have PEQ and I also recommend their subs but two will probably come in over budget unless you find used. Best of luck!

