definitely change all the fluids in any used bike, chances are that it still has the stock fork oil.
oil breaks down over time especially without use, condesation in the cases occurs and with out running the heat from the motor doesn't evaporate the water and the oil becomes contaminated.
Check the cable too, there might be a kink in it somewhere, then lube the crap out of it, lever pivot, and even the ball on the clevis.
I also noticed from your photos, that you are a rather big fella.
Springs should be your next investment. When changing the fork springs change the fluid and use a heavier fluid this will help the action considerably without revalving.
If it were me and have the $$$ at the time I'd get the whole set overhauled and Gold Valved with the proper spring rates. Race Tech is the place to go for all that. the kits are easy enough to install by yourself. but with the amount of wear on the rest of the bike I'd just send it off.
I know a guy here in Pensacola great dude, great work, fairly quick turn around. He did my buddy's KDX up w gold valves, springs and and a complete rebuild for $500.
man let me tell you,after I rode my buddys bike with the suspension work, I was very dissapointed with myself for buying the KX250, when there was a brand new KDX sitting next to it, , I had no idea how good it could be.
Oh well.
