I don't think "certified" kit has any useful meaning if the airframe itself is not legal to fly in IMC.
Avionics certification means so little. I have a KFC225 certified autopilot (cost probably £40,000 although it was never successful in the retrofit market) which has several major design defects, causing regular failures right across its user base of several aircraft types. It seems that, in GA, there is little or no design quality review and any old junk can be TSOd.
If I was flying a "homebuilt/sports" type I would equip it for flight in IMC and not worry about the avionics being certified. Anyway, one can pick up a used Garmin 430/530 or something similar fairly cheaply, and you get the OBS mode for flying DIY approaches
It's the airframe I would worry about. I'd want excellent electrical protection and bonding together of all the bits - not just for lightning protection but for protection from a static buildup in normal flight in rain etc. A deiced pitot tube is absolutely essential because pitot icing is perfectly possible in VMC, below 0C. Even a decent quality radio is so important - every time I fly I hear some ATCO tearing his hair out because some plane has a crap radio, the pilot knows it perfectly well, but evidently nobody is going to do anything about it.