Hi Seamad,
I have used Solarflux before and it does a great job for a lot of processes, however, for sanitary fit-ups it can't be used because it leaves a layer of residue. This residue is very hard to remove and if it's not removed can harbour bugs. Back purging is the best way but it does cost a whack more in equipment, setup time and argon.
A straight DC TIG machine can easily weld stainless steel. I use pulse frequency and duty cycle to control the width of the puddle and heat input. Stainless warps so easily it's not even funny. By keeping a narrow/deep puddle and controlling the heat input, you can get some amazing strong welds that are very obscure with little distortion to surrounding metal. If all I had was a straight DC machine, I would still give it a go. I'd just run short beads of about 25mm and let it cool, repeat.
As for cleaning, I think I've way over-thought the process and it will be quite easy.
Fill with 80°C cleaning solution. Fill to the top, let it sit over-night. Drain.
Pressure wash first through the cone side, then through the top side. Just a cheap Aldi pressure washer. Hand clean all the sanitary fittings.
put covers on all the ports, fill with about 5 litres of iodophor solution and give a good swish around. Set in the corner until it's needed again. Iodophor solution will stay active indefinitely absent anything that will absorb the iodine or react with it. Also, while resting, the iodine will gas out and keep everything sterile. Phosphoric acid based cleaners only sanitize what is in contact with the liquid.
If you really wanted to go all ballz and ensure even the hardiest of alien bugs are made dead, buy some bulk hydrogen peroxide and put a couple litres in at 12%. Nothing can survive that **** and it's no rinse as well, but that's serious overkill.