Equipping an IR/IMC aircraft from the ground up is quite expensive. I bought an aircraft I liked and then spent the next year and half bringing it upto IFR standards. It was a progressive thing with mode C then Garmin audio panel then the second nav/com then ADF and so on. The most expensive part was getting the kit FM immuned. You only need one of the nav/coms to be FM immune but I bit the bullet and had both done on the basis that the reason for 2 radios was to guard against failure and sods law would say the only FM immune would fail!
Other things like swapping all the instruments out for backlit was slower as I waited for them to fail first! Lots of other "minor" things like the fuel computer and connecting the handheld into the intercom all added to the time and cost but have been worth. There is nothing in my aircraft now that is placarded out of use or innacurate!
What I have now is a fully functional digital IFR equipped public transport aircraft that is a great ATPL hours builder for me.
Putting good kit in will help to ensure that it lasts, I have done nearly 300 hours in mine with only a couple of minor problems.
It also does not need to be new kit there is a guy down in the South East who sells kit from wrecked aircraft at a fair price. You get it checked by an avionics shop and away you go. This was where al lot of mine came from without problem.