This kind of 4" digit?http://www.kingbrightusa.com/category.asp?catalog%5Fname=LED&category%5Fname=KC1%2DDigit+4%2E0in&Page=1
I'd recommend the Common Anode parts, drive from 12V, connect the cathode segments to TPIC6B595 shift registers to drive them.
This board here can drive 12 digits, use 2 boards and wire up one TPIC6B595 for the 25th digit.
Probably want to wire the anodes together seperately and not source from the board tho. Connect grounds together.
I sell bare boards for $5 each mailed to US locations, all thru hole, easy to assemble. International, a little more for postage.
Complete kit, or assembled tested boards, I'd have to price up.
The actual counting down part is easy.
set up 25 bytes with 9 in each.
once a time period, test if the lowest digit is not 0 and decrement it.
If it is 0, set it to 9 and decrement the next digit.
if the next one is not 0, decrement it.
if it is 0, set it 9 and decrement the next digit.
Will have one big long chain checking if a digit is to rollover or not.
Longest chain will from 1000000000000000000000000 to 0999999999999999999999999
And you can add leading 0 blanking too.