What sort of car? If you are using factory parts to convert to manual you won't need to involve engineers. Same deal if you are just using off the shelf (Selby, Lovells etc) springs to lower it. If you were going lo-rider with airbags or hydraulics etc then you would need engineer reports. Body kits are generally just bolt on (or fibreglass/bog on!) and generally have no affect on performance so again no need for engineer. Exhaust shouldn't be a concern either as long as you haven't hacked holes in the inner guards to run your headers through (like some V8 conversions into early Holdens). Of course there are db levels that you need to adhere to for rego and the exhaust needs to exit a certain distance past an opening window (no side pipes on four door sedans), your local exhaust shop should know exactley what is and isn't allowed. The later model the car is the less freedom you have with mods.
If you are talking about doing a V8 XF (as your sig suggests) then you probably will need engineer reports on the conversion. There was a thread recently in the Garage section about XF V8 conversions - do a search for more info.
You know, somebody actually complimented me on my driving today. They left a little note on the windscreen. It said, 'Parking Fine.'So that was nice.