I would buy second hand ones to start with. Then you can try a range and find out what works best for you, then use your voucher to buy them!
I have mainly used Bumgenius, there are different types but the ones I have are aplix (like velcro) and get made up with 1 or 2 inserts inside, so when you put them on they are just like a disposable. It meant DH and nursery both managed with no difficulties! They are a type called pocket nappies, ie you open the outside up and stuff inserts inside. Other well-known pockets are Tots Bots, Fuzzi Bunz.
You can also get all in ones, or snap in ones, which are all made up and don't come apart. These are easiest of all to use (no need to stuff them or separate them for washing) but they take longer to dry. Best known brand I can think of offhand is Itti Bitti.
Then you get 2 part shaped nappies, which are the absorbant inner (mostly made from towelling material, either bamboo or cotton) and then a waterproof outer. In theory you can just change the inner and re-use the outer a couple of times, but in practice it is rare to get more than one change out of it. You can try Little Lambs, Tots Bots, Bambinex.
The quickest drying are flat terries, available in either cotton or bamboo (but I would only get bamboo as they still dry quickly but are way more absorbant). These need an outer waterproof wrap too and because they aren't already shaped you need to work out how to fold them! Def the cheapest and quickest to dry though.
If you want ones that will grow with him, get either flat terries or birth-to-potty ones - they are adjustable. My bumgenius fitted him from about 3 weeks and still fitted DS2 when 3.2.
As to how many you need, well I have a stash of about 20 (16 Bumgenius plus a couple of others) and with a 2.5yo that was enough to wash every 2-3 days. But then I am a lazy slattern who would happily leave him in a nappy another hour if it meant no washing today....so maybe it would be better to change more frequently than I did! If you are prepared to wash more often then you could prob get away with about 12-15 or so, allowing for ones he's wearing whilst waiting for the rest to dry.
We used disposable liners too, you can get thick ones which are washable (assuming wet only) so they were used a good few times. Never used our fleece liners so I wouldn't bother with them, esp if you are thinking of disposable liners anyway.
I used fleece wipes and a spray bottle (Boots travel type). Wash sep from clothes, although occasionally one found its way in with no ill effects! You can wash at 40 with small amount of detergent, no conditioner. When storing them waiting to wash, you need a container with a lid, just chuck them in dry - no soaking (unless you want poo soup!)
I would have a look around some sellers websites and read some reviews. Then try some and see how you get on!