I have one in my workshop 20x10 its a 5 KW off ebay £172 works very good and cheap to run https://www.ebay.co.uk/itm/12V-Air-Diesel-Heater-5000W-4-Holes-LCD-Monitor-PLANAR-For-Trucks-Boats-Bus-Car/292710819687?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
I use RTF road traffic cleaner its very strong and can fetch paint off but boy it can cut through grease oil etc. I paid £15 for 25 litres so its cheap just ask the hand wash guys they use it or pro valeting places https://www.ashfieldvaletingsupplies.co.uk/trade