I thought about it and have come to the conclusion that I agree with your opinion on the welds on the manifold. So that's a no-go!
So now I will either make it myself or have someone weld two 90 degree aluminium tubes together. The tubes cost around € 30,- and welding shouldn't cost much more if I deliver them cut to size I suppose. Another thought I have is to make them myself by gluing the aluminium tubes together. Glue is supposed to be as strong as a weld and much easier to do yourself. Another thing I found that might be interesting is alutite (
http://www.alutite.nl/). This stuff works like soldering but is almost as tough as a weld. It's something that supposedly comes from Sweden and is specially made to bond aluminium parts together. The youtube movie looks interesting anyway.
As for what carb to choose. I talked to a mechanic I know this weekend. He said that in principle any carb would work (they all mix fuel with air). Because the single carb needs to feed two cylinders you need more air than with stock. That can be accomplished by an aftermarket air filter, but using a bigger carb than stock besides that won't hurt either. That's probably why they all offer 40mm carbs with the conversion kits, as supposed to the stock 34mm carbs. That said, the guy from OneUp uses a 32mm Mikuni VM round slide carb and got his bike running by the looks of it. The only thing I know is that a smaller carb will produce a faster airflow which gives you "more" power at low revs and less at high revs. A 40mm carb is better at delivering more power at high revs but won't hurt the low rev performance either I guess.
Mikuni has VM carbs that are really simple. No things like accelerator pumps so less that can go wrong. But with those you have to be gentle on the throttle to prevent the engine from going lean on you. The TM (pumper) carbs have an accelerator pump that fixes this problem, as have the FCR from Keihin. The third option is of course a CV carb, but I wouldn't prefer one of those because they have a slight delay in throttle response and maintenance on them is more expensive if the diaphragm fails.
A shop in Germany sells new Mikuni VM36 (slightly larger than stock) for € 184,- and TM36 for € 255,-, both new. A Chinese carb for € 50,- or € 60,- would probably work too, but getting parts seems hard so you'll probably looking at buying a new one if a part fails. I've also seen reports of them being very hard to set up properly.
I myself am tempted now to buy a used Keihin FCR40. That's a pumper flat slide carb that has everything going for it, and it would cost me € 130,- including throttle cables and handle. Sound's like a bargain to me and parts are available. What do you think?