Hi and welcome
Lines-wise, no you don't need to spend a lot, but then again it depends just how good/bad the one you got off ebay is. Contact Pitsford Pirate on here for very good, very reasonably priced lines.
Rod? those Fladen rods seem to get good comments on here for a budget priced model but you might want to consider something a little lighter perhaps for stocked lakes, as an 8 weight is quite heavy duty, especially if you like fishing dries. Loads of good deals around on 6/7 weights.
Use a knotless tapered leader (with shorter length of lighter tippet tied to end) - or you can just use a straight length of mono, again with a lighter tippet added, which is what I do most of the time. There are mixed opinions on the benefits of tapered leaders, depending on the type of fishing, but personally I have never found much noticeable benefit using them.
To fish deeper you could also use a sinking line - a slow sinking/intermediate will be good for searching different depths on those types of waters. You could use that on your current 8 wt rod, and get a lighter rod with floating line set up to give you two options.
As Jim says, no need to jetison your gear just because it might be cheap though - if its working for you and catching you fish, thats what matters.