If your next finder you are interested in buying is L and regularly available on the auction, go with that. An F-105 is going to have a decay of 2.050 PEC per drop. eMINE FS (L) on the other hand is typically right around that same amount when you factor in auction MU, and is also has a 676m max depth compared to F-105's 522m depth. Finder range varies a bit between those two, but you'll want to pencil those out and figure out total decay per drop, decay per surface area searched, and decay per depth to see what comes ahead as an efficient finder.
Personally, I don't find unlimited finders worth it earlier on unless you are at a point where it's hard to find limited finders on the auction that fit your needs or if you're using enhancers (they last longer on unlimited finders). I've switched between using primarily unlimited and limited sometimes, but unlimited use is pretty dependent on what you're trying to do.