The best low end amp I've used requires only a screwdriver and some sort of case, I have one in an old wooden silverware case with its power suppy. I don't want to spam, but here is the easiest way to find it. Search Amazon for:
"4x100W @ 4 Ohm TK2050 Class-T Digital Audio Amplifier Board"
The reviews on Amazon are helpful.
I'm using this power supply, search eBay for:
"400W 30V 13A Switching Power Supply Current Control Charger LED CCTV U30".
The amplifier board and power supply together ran me about $110, the silverware box was yard sale junk, so maybe 50 cents
The output wattage ratings assume a 30 volt supply. The board will run on a single ended 10 to 30 volt DC supply, so it would be easy to use in automotive or marine applications. It's a Tripath chipset and sounds great.