If done properly a weld is stronger than the surrounding metal. The key is it needs to be done properly - as in, the welder needs to know what the hell they're doing. Bikes are tricky since (among other things) they often incorporate odd alloys that are difficult to match, have varying thicknesses of metal, etc. I'd guess that in many cases of weld failure it isn't the weld that actually fails (although there are probably some of those too) but the point where the metal begins to transition from weld / repair to surrounding material. Maybe the OP is a good welder. I'm an okay welder who can tack a couple of non-critical pieces of metal together and that's about the extent of what I need to know how to do. I'd never trust myself to do anything critical or on which my life (or someone else's) might depend without a lot of additional research, training, practice, etc.
I read up on this a while back and the conclusion I've reached is that for a road or hybrid type bike a conversion to disk brakes is not really worth the extra cost, expense or effort. On a mountain / trail bike it might be if you're constantly bombing down hills in wet / sloppy conditions. At this point in my life I ride road much more than mountain and my mountain bike skills are decent but not great to the point where I'd see much benefit. I don't take as many stupid risks as I used to (I get to take all new stupid risks!
). Anyway, I'd look for a new fork with the setup already on it. As has been said above, why do you want to risk it? Is going face-first into who-knows-what at 30-35 mph worth it just to look "cutting edge". The answer should be a resounding "hell no". I used to ride down around Newport / Laguna Niguel all the time and there are very precious few situations anywhere near there that require the extra stopping power of disk brakes.