Count on each Lok you run at the same time having 1 amp. 3 loks 3 amps, 5 Loks, 5 amps. Real usage is less, so you might get up to 5 loks on 3 amps, and more on 5 amps.
I strongly suggest you visit the JMRI.org page, and select a system that works well with JMRI. Makes it so much easier to set up decoders in Loks! (No fiddling around with CV numbers, JMRI is human readable)
For a new user, best to contact your mates running digital in your area, and see what they are using and it they can help.
Not sure what decoder brands Hornby uses (you did say OO guage) but on your side of the pond, popular brands might be Lenz, Loksound, or Zimo.
Loksound and Zimo have sound decoders, quite complicates (germans!) but lots of features to learn.
The wiring could be the same as you used for DC, but you need less blocks, and no kill switches for sidings. You still need to be aware of polarity, but except for reversing loops, everything has the same polarity.
Plan for blocks if you are going to use signals, otherwise 2-3 blocks with boosters should do for that size layout.
Lots of folks use at least one booster, and do not connect their DCC system directly to the tracks. Better for the lesser price booster get zapped by a short than the high priced DCC system.
A good starting system would be the SPROG. Made in UK, controlled with a PC and JMRI. And reasonably priced. And you can use your smart phones as throttles (We do that at our club layout).
Good luck, and remember to have fun!