Just a case of selecting the right tool for the job..
As the SPD-S has been around a while, they do pop up second hand on a well know on-line auction site

That might save you some pennies.
That said, a brand spanking new one will have a guarantee, and if you are UK based that's for 3 years.
The TMC-6 would indeed add 6 extra trigger inputs via MIDI, also factor in the cost of adding the triggers too. Price starts getting scary.
Another consideration, you will need something for the SPD-S to plug into to make it heard.
If you plan just to plug it into the sound system, fine. You might need a DI box, especially is a stage box / multi-core / snake is used. You will then need some form of monitor too.
If the band has a keyboard player, he might have a spare channel available on their keyboard amp.
You could use a bass amp, at a push (may well lack top end frequencies).
Electric guitar amps - not a good idea. They lack the 'full range' frequency spread the SPD-S can generate.
I think the first hurdle to overcome, is learning the SPD-S, or SPD-30 if the decide on that route.
Both will have a learning curve to overcome, and it's quite a big one as they are complex units.
If you haven't already, have a look at the v-drums.com forum. There's experts-a-plenty, and you may find answers to questions you hadn't even considered yet.
http://www.vdrums.com/forum/index.php?
Another 'heads up', download the pdf manual. It might seem information overload, but again it may answer some issues you may be having.
ftp://ftp.roland.co.uk/ProductSupport/SPD-S/
ftp://ftp.roland.co.uk/ProductSupport/TMC-6/
http://www.roland.com/products/en/_support/om.cfm?PRODUCT=SPD-30&iRcId=25862050&dsp=1