What kind of BOV should I get?

Due to the closed loop turbo system in our A4's you can not release the air into the atmosphere, you have to recirculate it back into the system. The MAF calculates air before the DV so if any air is lost after that calculation by releasing it into the atmosphere you will run rich for a little while. How damaging this is, is disputable, but on 2000+ models the car adapts to running a BOV because of the MAP sensor so you will only run rich for a day or 2. The BOV will need to stay closed at idle. A BOV that remains open at idle is not good for a car with a MAF and you will stutter and stall. Forge 004 and SSQV are both closed at idle and good choices. If you run standalone software (034 Motorsports carries it) you can run any BOV you want.

There are also hybrid BOV's that only vent a portion of the air. The most popular are the Forge Splitter, and GFB. The splitter is the easiest to install and run and the GFB is just a little tougher, but you can adjust the amount released to atmosphere for 10-50%. The GFB is ~$250 and the Forge is $175 and can only release 10% vta or full dv (should only be left as both since it releases more air).

And if you have a B6, they are equipped with a Wideband o2, stock. the ECU is programmed from the factory with the target AFR, and it will adjust fuel trims accordingly to maintain that, tuned or untuned. So you can remove your MAF and run any BOV you want. Won't work with b5's.
If you have a pre-00 A4 can you run a BOV? Technically no, but I run one on my 98.5 with no problems whatsoever. You only run slightly rich right after you "blow-off" and I can't see it doing any damage. Maybe a catalytic converter in the long run, but I doubt it.
**note: I take no responsibility if you put a BOV on your car and it fvcks it up.

What DV should I get?

It is entirely up to you. First option is the 710N. This DV comes stock on Audi TT (225hp version) to compensate for the added boost from the k04. It is the cheapest one you can buy (typicaly found for $35-$40) and will work fine on any k03/k04 application. It is not serviceable however. The next option is a Forge series DV. Forge 006 and Forge 007 (virtually the same thing but the 007 has a smaller external design for optimal fitment on an S4 application). They are fully serviceable and adjustable. If you are upping the boost all you need to do is switch out the spring. This (007) is probably the most widely available and recommended DV to get. It is ideal if you�€™re going to be upgrading the turbo. There are other DV�€™s out there like Bailey�€™s, EVO, Hyperboost, Stratmosphere, etc. They are essentially the same as the Forge series, so whichever you can get for cheapest is the one you should get. A new DV will not add any power but it will make the �€œpsshh�€� sound more audible, especially with an intake or air box mod.

More info on DV's can be found here- https://www.audiforums.com/m_531160/tm.htm