I installed the 7kw Triton T30i Hand washer. You can buy them for under £50 and are very easy to install. Just remember your wire loading (6mm min). The thing is for washing hands (obviously) and if you need a little hot water in a bucket or your sink for cleaning jobs (I regularly use it for cleaning our bbq and other messy jobs). It provides pretty much instant hot water (give or take 5 secs) But if you need a lot of hot water or a big bucket of hot water, you may be standing a while as the water quantity and pressure isn't your average household pressure/quantity!.
My father-in-law installed a boiler type water heater that sits under his sink. If like me, you're a hobbyist and not in the garage/workshop every day, then having an under sink heater thermostatically boiling away, isn't a very economic way of getting (instant) hot water. If you don't want the thing heating up to temp all day long, then you will need to switch the thing off and when in the garage again, remember to switch it back on (and wait for the thing to heat up - not so instant hot water). This again was another reason I didn't chose the under sink boiler system.
But like everything else, it's a personal preference.
Jonny