As I understand it (happy to be proved wrong
)
- If you
can't prove the age of your engine, then the latest (and most stringent) emissions standards apply. You'll need a modern engine (i.e. fuel-injected, cat-ted) to even get near a pass.
- If you
can prove the engine age, then they test against the emissions requirements in force
when that engine was made, e.g. pre-1992 engines don't require a CAT, pre-1960s (?) only need a smoke test, etc.
The bit about choosing a 'Q'-reg - you don't really get much of a choice.
If you can prove that nearly all of your cars bits are brand new (I think you're allowed one old component?) then you can get a modern 09 registration.
If you've nabbed all the bits from a single donor then you can transfer the donor's registration.
Otherwise you'll be given a 'Q'-reg. And just because it's a 'Q' reg doesn't mean you get an easy emissions test for the engine - see above.
Hope this helps.