Hi Stormcatcher,
I'm sure you've probably sorted this requirement by now, but I thought I'd offer a little tip.. that if your DM list is 3-4k records, it might be worthwhile getting it to a few records over the 4k mark.
At the 4k+ level, your mailing will benefit from being able to be classified as 'Admail' which carries the biggest discounts with Royal Mail. Also, maybe this is obvious, but A5 is quite a bit more cost effective than A4.
In terms of pricing, you shouldn't really be spending any more that the cost of a second class stamp on everything.. i.e;
Printing (assuming its an A4 letter or A5 flyer)
Folding (A4 letter down to A5 or DL)
Enveloping (inc the envelope)
Printing the address, return address and postmark on the envelope,
Posting by RM second class
In terms of security, ensure you smuggle in at least one 'seed' address to somewhere you have access to (your home or business address, or a friend's business) with a very obviously identifiable fake name, so you will know that if anything ever turns up at the seed address with that name, your list was sadly compromised and you know where the leak came from.
<Edited by moderator - no, it isn't allowed on this forum!!>
G