I've built / done a fair few single speed conversions and further to what's been said I would recommend:
1) try out your bike with all it's gears in place to find your ideal ratio, then use Sheldon Brown's 'Online Gear Calculator' to ascertain what size cogs you need...
http://sheldonbrown.com/gears/
the calculator is also useful as you'll see you can achieve the same ratio with various different sized cogs.
2) if your frame has vertical drop-outs, use a sprung tensioner as fixed ones do not compensate for slight un-roundness* of cogs and rings (the reason you will notice a sprung tensioner bobbing up & down very slightly) and as the chain stretches you have to adjust them (unlike sprung ones which adjust themselves).
NB - I've tried DMR, Mr Control, Gusset and homemade tensioners but have been most happy with On-One's Doofer which Planet X are currently selling for £8 online! I know it's a copy of something, but you can't knock £8 posted. Buy two in case you snap one, and add one more link than the chain actually needs
3) use unramped cogs and rings if possible / you can afford them - they engage with the teeth better - and once you have the right size cogs, fit a nice fat 1/8" chain - stronger and look er... fat!
4) if not using unramped rings, middle rings will sometimes not fit in outer position on your crank (and outer rings sometimes don't fit in middle position). IMO, cranks look like nicer with the outer ring position occupied so you can see the spider legs in front of your ring (if you follow me!)
5) don't be tempted to run bike without a tensioner (thinking you have achieved the 'magic ratio')... as soon as your chain stretches, you'll lose tension and have to start all over again, adding links and a tensioner etc. Also, I think the redundant mech hanger looks ugly.
6) make sure you have lots of thin spacers for your freehub - this makes getting the right chainline easier and is cheaper than buying a different BB
* can't remember the technical term
all the above are only my opinion of course
ps, TSP when you say serious budget I presume you mean low? If so, follow my sage *cough* advice and once your single speed is running sweetly, sell all your old drivetrain on here!
