Cold Machines are much more popular due to cost. Hot machines are necessary where lots of grease or grime have to be removed.
You usually get what you pay for. Lower priced DIY models are fine for small jobs where 1-2 hours maximum use is expected. If you are going DIY, buy a recognised brand name with available parts and accessories.
Again, you usually get what you pay for. The more expensive models have brass pump heads, quality hydraulic-type hoses and industrial steel or stainless steel lances. If you need to wash all day sometimes, go for a four-pole slow-revving motor model.
Electric driven machines are more popular due to cost. Engine driven machines are useful where electricity is unavailable, or where risk of electric shock is an issue. Additionally, with the bigger engine machines, higher performance can be achieved than 240v electric machines.