I DIY AC on older cars for sure. Here is what you need to understand:
1. If it's simply a leak - look at common places for dirt build up. If you don't see any - it might be one of evaporators.
2. Gauges useful for diagnostics, but in case of leak, I really don't use or care about what they show anymore.
3. Once leak fixed, you need to vacuum system and make sure vacuum stays. Easy enough. Use Nylog on connectors (o-rings)
4. Charge by WEIGHT. Don't look at gauges. There is different modes, temperature, altitude, etc. Don't bother. Just charge how much it should be. You know how much in a can, just eyeball last can, thats it!
5. Oil, Oil carried in a system, most of it in compressor and then it's spread out around. If you replace just a line - there is probably 1/4 oz oil in there - don't even worry about it. Because otherwise - you need to make sure you buy correct PAG or ester (whichever it is) weight oil. Too much oil is not good either.
So, as you see - no big deal. Leaks is easiest because all you do is find a leak, fix and vacuum/charge.
Only problem is - it's illegal to vent refrigerant to atmosphere. Shops have recovery machine. But if all your refrigerant already "lost" due to leak - you are OK