I don't have a whole lot of knowledge on this, but here goes. I have an old Murray Ultra mountain bike (prob. 15 yrs.+). It is a 15 speed bike, with a 5 speed freewheel. The whole Shimano system is the 515 which includes front and rear derailleurs.I really don't want to spend a lot of money on an upgrade, but an up grade of some sort is necessary because of simple wear and tear. All i'm asking is there a set,or equivalent where i can replace - at least the shifters to a more modern ones,and front derailleur without having to break the bank. At the very least the front derailleur and shifters.
Having restored a number of old cheap bikes, I can tell you that the biggest problem is almost never the derailleurs/shifters. Usually these bikes have not been ridden much and have spent most of their time in a garage exposed to the elements. The biggest problems are rusted parts, poor lubrication, and horrifically bad tuning. Frequently, on a bike like this you're likely to find that it was never properly put together.
Here's a quick checklist of things to go over that can be fixed economically:
If you have to replace the shifters or derailleurs, just try to find cheap replacements or used parts that are similar. You'll want to stick with 5-speed stuff, you aren't going to be upgrading to a 10 speed freewheel (they don't exist!).