I too have carried out a Mk3 to Mk3.5 Conversion. This is based on a 2006 MK3. I have only upgraded the front end at the moment, so I call it a Mk3.25.
I have made up a front grill of my own design and fitted DRL’s behind 2 small grills where the front fogs would normally be. I managed to pick up 1 headlamp on eBay and after watching for a long time, eventually gave up and bought 1 new one from Mx-5 parts. These are the ones with the silver reflectors which are fitted to the hardtop MK3’s.
I have also fitted an LED front badge which lights up when the car is running. Unfortunately this can not be seen as well during the day, but looks excellent after dark and draws a lot of attention.
I have lowered the car by 35mm and fitted RX8 wheels. At the moment these are fitted with 225X18X45’s which were on the wheels when I bought them. I want to change these to 35’s but as the tyres are almost new, it seems a shame to not use them. They clear the arches fine, but the gearing is slightly affected and the speedo under reads by about 5%. However I use the Sat Nav to keep a more accurate check on my speed until such times as I fit lower profile tyres.
Very pleased with the car so far, but it is still a work in progress and there will always be more to do. I attach a link (I hope this will work) with a few recent photo’s