A lot of boat for the money. Built like a proverbial sh*t brick-house.
On the engine side, interested to know why just 20 hours? Tohatsu are a bit agricultural but very well engineered in my opinion. Even if this was connected to a hour-meter I would take that with a pinch of salt, as I'm assuming it was on another boat at some point in 15 years?
Drop the leg for sure and get the impeller and gear oil changed before you do anything. The thermostat is on the top cylinder, and the housing itself is a good gauge of the environment it's been in. There should be an anode in there too to check. If the oil has any emulsification, then rebuild water-pump, and always fit new drain and fill washers.
You can also run a compression test on each cylinder for peace of mind. All cylinders should be the same in terms of compression, give or take a few PSI.
The leg should raise and lower and no weeping on the rams. Again you can check the fluid level. If it needs topping up use Mobil 220 ATF.
Before running it on muffs, take each spark plug out and squirt in some fogging oil. If it's just done 20 hours, those bores could be as dry as the Kalahari and you risk snapping a piston ring from a standing cold start!
I'd also be interested to know if fuel is in the carbs, or if it was starved until it stopped. There's a brass hexagonal fuel drain on each carburetor, so drain them. They should run clear, no sediment. If it's been laid up, then the fuel is potentially stale and you risk gumming.
There's a fuel bowl on the side of the engine, remove that and clear the filter and refill with fresh petrol. Also check the 2-stroke oil in the reservoir.
Lube all the grease nipples too.
Parts diagram here: Tohatsu parts for M90A