Dr Wyndaz,
I would add try an 8 foot single pointer (around £100)first to get used to the A ladders, before you go higher. I know some very good window cleaners who don't like extension A ladders but prefer normal extension ladders.
Get Class 2 Trade aluminium, they are worth the little bit extra and will last a lot longer than domestic ones, which are not legal for putting employees on anyway.
A good setup is: 8 foot pointer, double 3.5m Lyte ladder for residential, ladder regs now say best practice is no higher than 9 meters, which rules out 3rd floor, so if you want to undertake 2nd floor work use a stabiliser to be within Health & Safety regs, along with a double 4.25m entension, or triple 3.5m
A good accessory for your main ladder is a set of leg levellers.
Be safe!