Cayugad said it well- the Pro Staff seems to get a lot of reviews and is not expensive. If you want to go cheaper, the Simmons ProDiamond is a pretty decent scope for shooting out to 150 yards. I got one on sale for $40 and have been very happy with it.

If you get a new scope, you will need to re-bore sight it- but you can do that yourself. What I did was put the gun in a rest, take the breech plug out, then looking through the barrel, center the barrel on an object 25 yards away or so. Then, making sure the gun doesn't move, look through the scope and move your adjustments so that your crosshairs are centered on the same object. This should put you on the paper at 25 yards- from there you'll have to shoot and make adjustments.

I don't think you need 150 grains of powder- IMO that's just overkill unless that's what your rifle shoots best. If yours is shooting very good with 100 grains Pyrodex, that's what I would stick with. 150 grains won't kill 'em any deader...