Can't advise on the drains, but your well out on the gutter vac'ing side.
Gutter clearing and SFG cleaning is usually a 1 off service, you may get repeat work but once a year tops, and like carpet cleaning you'll find lots working off ladders doing a house for £15
But, like carpet cleaning their is always room for a quality higher price job.
Getting a gutter vac doesn't mean you'll be inundated with work, it takes a lot of time and money n advertising to get rolling.
You will also need a genny, if you go cheap on your gutter vac you won't clear the gutter blockages and swan necks so up the ladder you go...
Don't get a back pack, waste of time if your only doing SFG cleaning, but a decent full carbon pole 25 ft is ideal, then make up some connector to fit the mains water supply, this will save you cash on unsuitable equipment, hassle, and time per job
HTH
Darran