1st of all i think you got a fair price, not the cheapest i've seen, nor the highest, but a good, fair price. you will love it.
there are an endless number of mods you can do. i'd recommend getting new tires. if they are original they are over 25 yrs old.... they are dry and liable to crack and give out. the front stock tire isn't very good off road, and you can find a much better one. check the new post by spud rider: https://tw200forum.com/forum/general-...elections.html
is the front seal you mention the fork seal? minor issue but need to get it fixed.
55 mph seems a bit low for the top speed, though there are so many factors that could effect that. is it stock gearing? (14/50)
first get it in mechanical shape and then ride it and see what you like about it and what you might want to mod.
oh and cycle racks rock, nothing bad about that choice.
enjoy that bike!