The only cheapest sealant I can find now is
Menzerna Power Lock Ultimate but its only 250ml. It can do 3-4 coats/times on medium size car.
Next comes Poorboys EX-P and Poorboys EX. One is pure sealant and the other is sealant+carnauba wax respectively. These will cost you 1.5k but then c845 is VFM compared to these.
The oil from Megs UP will go away after you let it haze and buff off. Some use IPA to take out polishing oils but I don't like to use something in-between.
As of applicator pads, checkout KMH pads in Amazon. I believe they are good, though not comparable to Megs.