If your fluid is "dirty" and dark, thats overheated, oxidized, worn out fluid. The slipping is probably a combo of that and the filter *probably* being clogged. At that point, personally I don't recommend changing it yourself because its more beneficial to have it flushed. You might want to put a can of Seafoam Trans Tune in the trans about 100 miles before you take it to be flushed also, I know that cleared up alot of sluggishness mine had. It clears out any built up varnish and crud and basically dissolves it. That trans is probably fine, just was probably driven pretty hard by the previous owner and you're certainly in the zone for having it done.

Leslie...I'd say at theleast a transmission service (pan drop with filter and fluid) is a minimum.