power door locks shouldn't be too difficult for the mechanically inclined. power windows are the same IF you have a donor car with power windows to steal some parts from (door panels, motors...).
i don't know what else would be power. do any cavy's come with power seats? if so, i'd imagine the wiring for them is under the carpet in all cars. i found wiring for power locks/windows in my '90 without either.
Power options should not effet anything if installed correctly. For the most part they are all momentary things. The power windows only have power to them when your pressing the button, same with the locks they only activate when you press the button. Sunroof same again, only under power when your holding the button. The A/C is different. Your A/C compressor is being driven be your engine belt. This is why you seem to be going slower when the air is on, the engine is working to turn the compressor.