If this is the larger barrel you've got then you'll need to pick up a grill rack for a 22.5" BBQ. I've not read all your post so I'm not sure what type of lid you have (was it a clip lid?). If not and you've had to grind the existing one off then look our for a cheap 22.5" kettle BBQ. eBay sell new ones for around £27. Otherwise, look our for a second hand one. I picked one up via gumtree for £15 (Lanndman model). I've used the grill rack, the lid and the handles one the UDS.
For valves and bolts etc, I've found Toolstation to be the perfect one stop shop.
Try these for the brass nipples (for vents). These are the 3/4" size so you'll need a 1" hole cutting - I have three:
http://www.toolstation.com/shop/Plumbing/Compression/Brass+Hexagon+Nipple+34/d20/sd2697/p63570One ball valve:
http://www.toolstation.com/shop/Plumbing/Valves/Lever%20Ball%20Valve/d20/sd2696/p14624Then you'll need a 1" hole saw with arbor (again, these are available quite cheaply from tool station).
http://www.toolstation.com/shop/Power+Tool+Accessories/Holesaws/M3+Bi+Metal+Holesaw+25mm/d80/sd1530/p56846http://www.toolstation.com/shop/Power+Tool+Accessories/Holesaws/Holesaw+Arbor+Hex+14+30mm/d80/sd1530/p42954This gives me four vents, one of which is the ball valve for more precise air control...but magnets work just as well - depends how neat you want the UDS.
The bolts I used were (I'll have to double check) stainless steel M6 x 50mm (I'll double check that as too short and you're grill won't rest on them, too long you'll have to cut down). You'll also need stainless steel penny washers to tighten these up inside (prevents leaks).
What are you doing for the charcoal basket? Those washing machine drums are perfect.
Have a look at my build and ask as many questions as you want. Nice little project which should take a day or two depending on your time.