1) Maxx hop units. I have a couple and they're great but if you don't NEED the extra funtions (built in tracer, BB retainer, flat hop/R-Hop nubs and arms) then a cheap ZCI rotary hop unit or even the standard one will be fine. The Cyma hop unit is pretty decent. If you're using the Maple Leaf rubber you need to use the Maple Leaf Omega nub, they're designed to work together. The shaped nubs that come with the Maxx hop unit are machined aluminium and are designed for R-hop/flat-hop and they're too hard for the ML rubbers, they tend to make the shots pretty inconsistent!
2) BLS ammo is decent, I've used some myself and quite liked them.
3) Barrels. Prometheus EG are very good, I have a Lambda barrel but have yet to fit it so can't comment. I do like the ZCi barrels as they're good quality and priced well. The main thing to remember about barrels is that it's more about the quality of the bore (how smooth it is, how accurate it is, how straight it is) than anything else.
3) Cleaning. Yes, standard kitchen towel will work as long as it doesn't fall apart. Cut a strip rather than tearing it so you don't leave loose fibres behind. Better would be a Jeye Cloth https://www.amazon.co.uk/s?k=jeye+cloths&adgrpid=104274814512&gclid=CjwKCAjwn6GGBhADEiwAruUcKgiUOoYT2ZBItbEu8ffgmtmH3YcYwAdXh6wzYZPXBwzvuwWTTSC8hhoCiIMQAvD_BwE&hvadid=447271061678&hvdev=c&hvlocphy=9045742&hvnetw=g&hvqmt=e&hvrand=4483414666538663342&hvtargid=kwd-317469785021&hydadcr=28146_1752652&tag=googhydr-21&ref=pd_sl_5j462045qj_e_p49. Alcohol is fine for cleaning, any half decent silicone spray will work for lubing your rubber but make sure you don't overdo it and make sure you don't get it on the inside if you can help it! Don't use WD40 though.
As ever, if you're going down the DMR route then this is a must read https://www.reddit.com/r/airsoft/comments/5g58ia/holy_grail_dmr_guide_updated_for_20167/ Good consistency shot to shot is the key.