Radio(s) up top as close to eye level as possible. Otherwise you will struggle to read the small numbers, especially if you have to lower your glasses to see them.
If money is tight and you are on a budget, a good dual watch radio is pretty much as good as two radios.
Hook up some switches for interior lights as it's easier now than later. Fit a solid led strip light on the roof, they sell them at Jaycar for caravans and the like. Great for in the hangar where it's dark and if you need to work on your aircraft or find something on the floor.
If there is a built in instrument light available such as in the ASI or compass, make sure you connect it.
Missile switches for Rotax mags or a high quality keyed ignition.