Perfectly straight forward (usually). Be prepared for the corrugated burner exhaust and air inlet pipes to disintegrate when trying to remove them. You will very likely need to replace those. The wiring is straight forward. (
assuming a Red Propex 1600 compact ?) There is is the main heater, the (attached) "black box" (with "low volt" and "burner lit" LED's, the thermostat unit (with rocker switch and rotary heat control) plus stuff like warm air out grill and ducting. Make sure you get the alloy brackets that hold the main unit to the floor as they "raise" it above the floor a tad, to allow cool air to circulate.
Wiring colours from main heater (on mine) are
Blue = permanently live.
Black = earth.
Red = to the stat, this is the wire that, when live, fires up the unit.
Orange = to the stat, this wire, when live, operates just the fan (no burner) for fresh air circulation and / or cooling down the unit after turning the burner off.
The terminals on the stat are marked easily for +ve and -ve but as for the other 2 (in the middle) I cannot remember which way rould they go, but no harm will result if you get them wrong as one is for "fan only" and the other is for "fan + burner" so a quick experiment (50 / 50

) will tell.
Some models have a "cool down cycle" that, when you (or the stat) turn the burner off, causes the fan to stay running for a while, to cool the insides down. Mine does not do this but Propex say some versions don't and not to worry.