Depend on what height you want to go to and your budget.
www.gardinerpolesystems.co.uk/acatalog/Industrial_Vacuums_and_Fittings.htmlI have the 640M Its the same model as supplied by Omnivac.
This works great even at 60ft and as it has 3 independent motors you can reduce the power when doing lower heights but have extra oooomph if needed.
The 515M is fine if you are sticking to standard domestic but personally I would say go for the more powerful model as better to have to much power available than not enough.
38mm hose is £60 + vat for 10m length which you can cut a section off to make your "U" bend. The 2 clamps and threaded rod are about £10 from B&Q and a roll of BLACK gaffer tape is only a couple of £.
With regards to the hose you can use cheaper or go very expensive. While making mine I used some really cheap hose which works ok but it does pinch relativly easily so it will be changed when I can be bothered to order some from Gardiners...lol
So for about £300 - £550 + vat (depending on vacuum and hose) you can convert your SL2 base sections for use as a Gutter Sucker.
Then just add some more base sections as and when you need to go higher for both windows and or gutters.
Regards
Mr H