Good place to start
http://www.guttercleaningsystems.co.uk/
The ability of the vac to do the job isn't totally dependant on the size of the motor/motors.
We went smaller as it isn't a service we go looking for any more. We have a 1500 watt single motor unit and that works fine in the majority of cases. There are times when we still have to go up a ladder.
If you fill your window cleaning bucket with water, dipping the end of your pipe into that usually clears any blockage. (We have only had 1 issue where we had to disassemble pipes to remove a piece of stick that jammed in the pipe.)
I don't know how others manage but cleaning the vacuum and pipes out when we get home takes me about 20 to 30 minutes on the front lawn. I hate that job.
Clearing gutter with a vac is certainly safer, but prearranging for a power point, set up time, vacuuming time, packing away and final clearup probably takes longer than doing it manually.
We have a camera system but the inbuilt camera battery won't do 2 gutter cleans. The battery is usually pretty low on charge at the end of the first clean. So if this was going to be a regular add on, then we would have to consider a better camera and monitor. (The camera is great and the pictures are also excellent; its just the longevity of the charge, especially the one in the camera just isn't good enough.)
Our setup didn't come from the company in the above link.
I have a skyvac (£650 and one motor) and no camera. At first I thought I should have bought the camera but now having done maybe a dozen or more jobs I've realise that you can tell if you've got all the muck out the gutters by how the vac sounds as you go along the gutter.
For the first 6 or so jobs I checked afterwards using a ladder and was surprised at how clear the gutters were.
As for long lengths of grass or other plants growing along the gutter, it's not a complication or something that's easier with ladder. Just watch any of the many youtube videos about how best to deal with that.
It's the same as with wfp, from the ground is much safer and quicker.
I charge the same for clearing gutters as I did with when using ladders and it takes a fraction of the time. In fact sometimes I now do the job slowly because I feel a bit embarrassed to go round a whole house in under 15/20 minutes and charge £60 to £80. Including setting up and packing away maybe 30 to 40 minutes. Including chatting to the customer, maybe an hour. If the customer is not there and has an outside power point, 30 minutes max.
I've also yet to have a problem using the customers power point.
And yes, for the first few jobs, setting the vac up did take longer than the job itself but now having learnt what works and what doesn't it takes no time at all.
What is a bit of a chore is emptying the vac out and cleaning it, to stop it smelling, and cleaning the filter but even that takes less time each time as you learn what works best and what doesn't. To clean the pipes/tubes out, just suck up a bit of clean water. Again, maybe watch a few youtube videos. There's enough of them that covers almost every problem you're likely to encounter.