Unless you are comfortable with refrigeration, I suggest you get a shop find the leak and fix it, and re-charge it. 1800 is very steap, of course it really does depend what parts you need.

If the system had a big leak years ago, you likely have plenty of moisure build up in the A/C lines and components. You definately need a new dryer no matter what, as well as expansion valve if it is original. Hopefulyl for you the compressor is still ok. Also hopefully the condenser and evap have not corroded internally and formed leaks.

It also really depends on how solid and long lasting of a system you are targetting for. You should assume that a half-assed job could very well not even last the season.

Note on thing, AC does not "consume" refrigerant. If you are low on refrigerant, as most likely is the case, it has leaked out, hence you have a leak. You are still running R12.

Also note another thing, R12 does not leak out as easily as R134, which has smaller molecules. This means that a system running R12 could/should last longer (charged) than one running R134, in the case of a tiny leak.

Also, your car is designed for R12. In the case of a conversion to R134, without an evap and condenser replacmeent, and maybe even compressor, you should not expect the R134 to produce super cold air, as would the R12. No conversion will definately yield a better performing system, at the cost of R12 though. A "proper" R134 conversion should not cost you less for sure though.

Again, it really depends on what you want from this system. All in all you should not expect a good easy way out, this is why most old cars have n ofunctional A/C.

I converted my 240 to R134, it has been leak-free but at the expense of performance. In traffic, or in extreme heat, it's not great. On the highway, it's perfectly fine. I did this due to the low cost of R134, seeing as most components are old, I should expect some new leaks to develop every couple years at minimum.

Good luck,
Greg