Well first of I suggest you take a look at my sheet here
http://gmrmedia.com/dolph/dolph-in/dvd2.html
it might not be the most comprehensive but yet it lists the best editions of Dolph movies. Editions here will be listed either on their technical or extras features merits, usually both.
So otherwise I say
- Agent Red US region 1 DVD (make sure it's the one with audio commentary because I believe another cheaper version came out later)
- Retrograde Region 2 you can go for either German, Dutch, (both easily findable on ebay or amazon), Japanese or French (although this one's less easy to find)
-The Defender Region 1 US (original aspect ratio but not 16/9), or Region 2 German, French, Greek, Italian...
- Diamond Dogs Region 2 German
if you need more help finding these you can ask, I tracked and imported DVDs from all over the world!