Nice to see another Necchi & BMW owner. I have three now (two SuperNova and a Mira) and my engineer friend in Seattle who has two non-E9 BWMs has four Necchi.
One vote to keep your car it's yellow color is the fact that it's been that color most of it's life. It probably wasn't custom ordered Polaris, that was simply what it was on the lot. The yellow color meant enough for a previous owner to go to a lot of expense to get it repainted a new color. You can honor that choice and history or you can make it yet another silver car like so many others. But it's your car, you get to decide what color you want.
You might have a ZF transmission, or less likely, a Getrag 4 speed. The ZF has a solid body back to the shifter which gives it very nice shifts. The Getrag has a steel metal arrangement for the shifter bolted onto the back of the transmission. You can see this from below if you lift the car. Be careful and don't lift by the rockers! They must be tested to see if they are able to support the car. Most E9 at this age can not.
The 5 speed Getrag 265/6 has an overdrive 5th gear and is highly sought after as it was used in a number of cars including the MB Pagoda 280SL as well as several later BMWs. It is the most reliable and strongest transmission used in BMWs. Rebuild parts are scarce, some NLA though some private hoards exist. Metric Mechanic has made new parts they say are more robust than stock and so they can do rebuilds. But they are quite expensive since that's a lot of custom work. I've seen used 'known good' 265/6 go for $1200 with a fair number of miles on them.
I was lucky and found my rebuilt one on ebay because I had a search running and it was posted with a great 'buy it now" price so I bought it less than an hour after it was posted. They guy had bought it years ago for a project but didn't use it and I'm sure didn't realize that prices had gone up so much in the meantime.
There is a lot of information on the forum about the 5 speed conversion as well as adding AC. The search tool is your friend. Welcome.