I wouldn't bother brazing the freewheel. You'll be converting it soon enough, so why go through the extra effort? Keep the freewheel for your parts bin. Regardless, good choice for a conversion.

If you think you'll ride this bike for a while, be wary of the stock wheelsets people are flipping -- they're ditching those wheels for a reason. They're alright for the short term (and I'd wager half the folks "upgrading" would never ride past the life expectancy of those wheels anyway), but thanks to fixxxxxie trend price inflation, many of those basic wheelsets are way overpriced. Unless you get a really amazing deal on the stock wheels, you'll likely get a lot more for your money on that Mavic/Formula setup.