You did VERY good for a rifle. Knight products are good equipment.
To start you off, plan on using 300gr sabot bullets, you'll find excellent accuracy with them as long as you don't push them too fast - 80 to 100gr of 777 max.

I put a $70 Bushnell Sportview shotgun scope on my Knight 3 yrs ago and it hasn't missed a beat yet nor needed re-sighting at all................... and I shoot it a bunch. Make sure you get a scope with a little extra eye relief and for muzzleloading purposes, you don't need THAT much magnification. Mine is a 1.5X4.5 I wouldn't mind having a touch more - for me 2X7 would be perfect. I've heard nice things about the Bushnell Elite 3200 series of scopes............a LOT cheaper than Leupolds, but maybe a tad more than you were hoping for. For me, if I had a more expensive rifle than the $150 USAK..........I'd be looking REAL seriously at the 1.5X4.5 Bushnell 3200 with the Firefly illumination.