Get a Bright Boy (from Walthers) for cleaning the track. You shouldn't need a wire brush.
Get some isopropyl alcohol and a soft cloth, and clean the wheelsets on your locomotives.
Kadee couplers are $4 per pair of 2 or $38 per 25 pair, which will do 2 or 25 cars, respectively. Provided your cars are Athearn, MDC/Roundhouse, Bev-Bel, Walthers, Accurail, Atlas, and some Life-Like, and body mounted, then you'll just need the #5 or #14 for standard head, or #58 for scale-sized...I prefer and use the #14s.
I recommend updating your wheelsets to metal ones. Metal wheelsets (especially the ones with metal axles) roll better, and don't get as dirty as quickly (thus, not getting your track as dirty as quickly). Depending on your rolling stock, trucks could be replaced too. I like Intermountain wheelsets, but Reboxx are good, and come in different axle widths.
Pick up a Kadee coupler height gauge so you can make sure your rolling stock works and doesn't come uncoupled when you don't want. Pick up a NMRA HO standards gauge, and you can measure several things, including your wheelsets if you don't immediately change your wheelsets out.
You can perform the process over time, as well as check and see if you really want to upgrade some of your older cars. Anything with couplers mounted on the trucks (Tyco, some Lifelike, Bachmann, AHM, etc.), is, unless it has sentimental value, not worth the trouble.