Try
www.ReefCentral.com Or
www.bareefers.org
The best thing you can get is a quality skimmer look at ASM G3's. Not crucial but they will make your like easier. If you just wanted to make it salt add the salt, get some seeding live rock and use your existing filtration. But if you don't have a skimmer you'll need to do ffrequent water changes, not just Freshwater top offs.
Also get a large bin, and keep 50 gallons of Saltwater on hand mixed at all times. It's really not a big deal.
Three things to remember.
1. Check your tanks salt levels weekly
2. Add freshwater top off RO if possible
3.Do frequent water changes like Fresh, not as much but weekly, maybe 10% if hte fish load is normal.
Hope this helps a bit PM me if you have more specific.. I did hte exact same thing you are trying a year ago, learned alot. Now I run 100 (Fish only) & 40 gallon Salt (Reef); 220 & 40 Gallon Fresh. And I'm soon will be bringing a 180 gallon reef tank online
Better mix your salt at home.. I'd hate to have to drive around London with 50-100 gallons of saltwater in my car

tiny streets having to give way with sloshing water doesn't sound fun. Great town go there a bunch.