Think about some training as stone can be hard to master and you might end up learning the hard way if you try and do it without training.
Call Russel at tile doctor ..
www.tiledoctor.co.uk To get you going on the slate for now...
Use pro clean from tile doctor as a cleaner. If you have loads of sealer you might need to use a solvent to remove it... Remove and go from tile doctor... A wallpaper steamer can be handy...
If you look to make tile and stone part of the service you offer ... Invest in high pressure machine and get a sx 7 and sx12 from ALLTEC
I usually clean on day 1 and go back on day 2 to seal...
Use 3 - 5 coats seal and go from tile doctor as it wil bring out nice color in the slate otherwise natural finish impregnators can leave you with a dull finish
Here is one I did
http://artofcleanuk.blogspot.com/search?q=Slate Hope it helps
Regards
Pierre