Unless you can find the 3.0mm x .35 tap, you need to break down and just buy the Rolex tap, like from CousinsUK. Sometimes they want to sell you the pair of taps- including the smaller one for the 5.3mm tubes. Or you could go to Jules Borel here in the US and just buy the single tap for around $35.
You don't say which MBW you are working on. On my Euromariner, which is an MBW, I removed the old tube and just tapped the hole and installed the new tube. Countersinking is a decision you have to make on your own. I did have to countersink the two noob cases which I put Rolex crowns and tubes in.
On older MBW's, when you removed the tube, it was also necessary to redrill the case hole so it was larger, before you tapped it. I made the mistake of not drilling the hole and busted a tap in the case hole- ugly, and a waste of a good case.