This operations only affects your hot water. You should also remember to un-plug your tankless water heater (you don't want it to kick on while you are flushing it out!) And, what ever you use, remember it's in your drinking water, so make sure it's clean.
I used 1/4 hp motor without any problems. It was $55 and while the output port was 1 1/4" wide, it had a cap on it that reduced the output side to work with a garden or washer hose. The pressure didn't seem to be a problem and volume was very manageable.
I first flushed the tank with clean water, using a clean kitchen trash can (13 gallon size, I think). It's taller than a bucket and takes less water to fill over the top of the sump motor. There are no switches on sump pumps, so it just goes on when you plug it in.
For the second pass, I pumped all the water into a second plastic tub and dumped it out. Then I filled it back up with white vinegar, which is about $2/gallon at the grocery store. I used 4 gallons. I ran the pump for an hour.
The vinegar is fairly weak acid, it won't hurt you or your pipes or you pump (most pumps have plastic and/or hard rubber impellers, so the acidity and the shot time you use it, is not important.
Remember to flush again, with clear water. If you just drain out the vinegar, you may have get cloudy hot water and you will detect a little of the left over vinegar smell. It's harmless and all goes away after a shower or a washer load of clothes.
Main thing is, you can buy the parts for about $70, including new washer hoses (4-5' long is plenty).
Last year, there were dozes of kits to choose from. Too many of them included disinfectant powders and chemical.
The year, their were very few kits to choose from and they average $150 each.
The kits are fine, but all you need is a pump (1/6 to 1/4 hp). I'd buy a new one to use, because if your sump pump is as nasty as mine, I doubt you'd want to drink water after using it.
I use 100 % white vinegar (5% solution). There are stronger vinegars (9% and higher), which you don't need, but they work about the same.
(Last year, someone recommended diluted vinegar on one of the web sites. That isn't right.)
As for frequency, that depends on the quality of your water. If you have a water softener or a household sized charcoal filter, you don't need to do this step very often. Annually is good, but every 6 months is too often. If you have that kind of scaly water (like we do in Texas), you should consider additional water filtering, too.
I love my tankless heater. It's been so much more reliable than my 50 gallon tank, which had little scale, but after 15 years, it just started springing leaks everywhere around pipe joints.
There is also a thread going on about scale deposits and how to control them. Scale likes copper, but I've seen it in brass and even plastic supply hoses. I've also heard it can be extra hard on houses with mostly plastic pipes, where it will build up heaver in the few places that copper is exposed.