Thanks for all the replies. The extra information you have provided has helped me make the decision.
I have a reasonable amount of IFR time, but I have not been current for quite a while
So this is strictly a VFR RV6. The main reason for the AH and DI is my piece of mind. This is why I want to do the cheapest installation I can (bearing in mind our dollar only buys 65-70 cents of yours, then our Government puts another 12.5% GST (goods and service tax)on top (we call it grab snatch and take:

)
From the information I have I would need to install two 4? venturi one to power the AH and the other the DI. (Any further advice on this would be gratefully accepted)
Can these be interconnected and still produce enough power for both instruments?
Yes they would look ugly on the outside (I had thought of installing them on the fuse between the legs) I guess an appropriate place for a relief tube. I never thought of fitting them inside the cowl I have enough room there. The thought of adding another 3-4 knots drag makes this an easy choice