IFR in experimental/homebuilt - new to homebuilt world

[Marc Zeitlin]

... if I build a a homebuilt from kit, what are the requirements for it to be legal for IFR operations?

In most cases, the Operating Limitations (issued with the Airworthiness Certificate) for the aircraft will state something along the lines of:

"Unless equipped for night and/or IFR flight per CFR 14, part 91.205, this aircraft is restricted to day VFR flight only".

So, basically, if you get the AC for the aircraft and then equip it per the requirements of 91.205, you're good to go. Some aircraft obviously make better IFR platforms than others, but the FAA is not cognizant of any of that information, generally.