Nikasil plating seems to be a real gamble. I've heard good and bad reports about Langcourt and Aptec ( www.poetonaptec.co.uk ) another UK company.
I've had plating done by Langcourt (although through a 3rd party) and it wasn't very well finished. The plating was flaking off round the ports.
Then again on the flip side, I've spoken with a few people who've had nothing but good experience with Langcourt.
My worry for you is that shipping from and to S.A would be expensive and if it does go wrong you'd have all the expence again in returning the barrel to them to sort out.
One option could be to send the barrel to Ian Emberton (www.iane.co.uk) Whilst he doesn't do the plating, he does personally finish off the ports etc so will check for a good job. I'm not sure of price, but it was around the £150 mark a few years back. Ian is well respected here in the UK on all things RS250. I know a few NSR-World members have used his services over the years, including me and he's been nothing short of a diamond.
Hopefully some of the other UK NSR crew will chime in and give their opinion???