Judging from your photo I would say that you are looking at check valves for vacuum lines. Nothing complex or expensive there.
The reason for taking the float out first might
be the relative ease of doing it from the top rather than dropping the tank slightly and then working blindly above it. Most of the tanks I have pulled required a person to take off the straps and then while supporting the (hopefully empty) tank on your chest reach around the bulk of the tank and blindly fumble around above the tank to take off the lines and electrical connections. You are lucky that it was mostly empty, doing this with even half a tank is terrible. Consider putting the exhaust (pressure side) of a high powered vacuum cleaner or air compressor at the tank's filler neck (seal around the hose with a rag to pressurize it) and putting a catch can below the outlet to get all the gas out. Dropping a tank is an unpleasant job, to be sure. Usually manufacturers give a person about three to six extra inches to let the tank drop down and work above it. Be careful not to let it drop too far as it will tear off the lines and wires or worse yet, kink a tube.
I cannot say whether or not this actually applies to your specific car, I have not worked on one yet. Anyone here care to chime in on the tank removal?
Do not give up over that! If it makes you feel the car is more worth the investment, there is a 1981 450 SLC that looks almost exactly like this
one a couple of blocks from my work and the asking price is $10,500.
On the other hand, there exists the saying that there is nothing more expensive than a free exotic or unusual car.
Trust me, if you fix that car and your son is mature enough to take care of it and appreciate it while not beating it to death - neither of you will ever be sorry. After it's road worthy again you ought to consider getting even a cheap paint job on it. This coupled with teaching him a proper regiment of cleaning and detailing the car will lead to him taking a great and deserved pride in his German coupe.
Oh, and please
encourage him to only make tasteful modifications. I know that even some of the smaller ones I made to my first car I came to regret later. Teenagers are exuberant and enthusiastic, but not logical.
...or you could send it my way. I would even trade him another car.