I did no training 
I had a quick read of the PADI manual and I'd dived with people in drysuits and spoken to them quite a bit, so I just bought mine and got in.
My first dives were in Blue Lagoon at 6m depth, with 2 buddies watching out for me, I did a few inversions on purpose, then a few more that just happened, but I coped with the forward rolls. I eventually went deeper at Capernwray and had to deal with the expanding gas in the suit.
Do I wish I'd done a £100 PADI drysuit course? No
Would I recommend others do it the same way as me? No, it was right for me and as I was doing it slowly with buddies I trust.
If there had been a £25 option I would have done it just to have the extra security of an instructor there just in case, but I'm not sure there is much more they could have taught me that my buddies haven't.
Arfie
I have the body of a god - Buddha