fatjoez - i like your idea.. I was thinking of doing the same thing in the future...
Personally, and this is only my opinion/preference I would rather supercharge an IS250 then get an IS350 especially if you got the IS250 for a good price which you have...also very unique!
I wouldn’t worry about getting the engineer cert. 90% of import cars out there are not, and who would really suspect anything in IS250 unless you are being a fool which in that case you can be driving and excel and you will get pulled over...same thing with the BOV. In my previous car (Nissan 200sx) I had a lot of work done and including a loud BOV and never got pulled over for BOV, other stuff YES..lol Also the harder you push you are the louder the BOV will be so if you are cruising along next to the boys in blue i really doubt it they would pick up on it...
You can go all out and upgrade the brakes, suspension, tyres, wheels, gearbox, exhaust system…etc etc
But in reality in my opinion all you really need is a 3inch straight through exhaust system and an ECU which you get if I remember reading the Stage 2 package.
Sooo to me a GREAT idea…hope all goes well for you mate…keep us updated!!!!
P.S. Would love blitzin’ an IS350 of the line…they always think they are soo much better and faster standing next to you on a set of lights!!! LOL ;)