Well, I'm no Debian expert, so you believe this or not at your own risk, but...
On Ubuntu, you can simply download any of the debs and install it - the OS version doesn't actually make a whole heap of difference because there aren't really that many OS dependencies at runtime (if any) - the OS versions give different card/tuner support (kernel), but that's before you get to tvheadend. So, you can just download the 32-bit or 64-bit deb and install it.
I am assuming that Debian is the same - they're close cousins, so I can't imagine why not.
In terms of which deb, the higher the number (later build), the more recent the code. There have been some significant changes recently, so youmight get issues with the very latest ones, but something around 3.9.438 should be pretty stable if you want to play it safe.
The next aspect is 32 vs 64-bit... drop to a command prompt and type uname -a. If it comes back with something about i386, you want the 32-bit version, which would be labelled precise-i386; here's the shock, if it says amd64 then you're 64-bit and need precise-amd64.
And finally, in each entry you have a debug version (tvheadend-dbg) and a regular one (tvheadend). You want the normal one.
Soooo.... assuming you're on 64-bit and want the last stable version before the most recent updates, you want this file:
http://pam.lonelycoder.com/file/4749b2c13a57793d63c0d5c2be61f8dc0d110062
and if you want the very latest, you want this one:
http://pam.lonelycoder.com/file/142819b896b68664954f758a4f6ed73fdc6ae63c
To install, use dpkg:
sudo dpkg -i <the downloaded deb file>
e.g.
sudo dkpg -i tvheadend_3.9.470~g6482d33~precise_amd64.deb
... for the 64-bit, latest build.
You can also add a repository so you get the very latest builds whenever they're created - have a look at
https://tvheadend.org/projects/tvheadend/wiki/AptRepository, but you'd be adding unstable unstead of stable - and would still use wheezy because that's what it's called.