[0] Not strictly necessary, just easier and less expensive. The Yoshifab parts make the swap way easier and cheaper than using a AQ171 / 251 DOHC Penta-type setup, so there's 2 schools of thought. Penta timing belts, etc, are different to Yoshifab kit belts and etc. AFAIK, the cam and intermediate shaft gears on Pentas are also different to B234 (well, the Penta gears have different part numbers to the B234 stuff).
[1] You can, but you're better off getting aftermarket pistons and rods. I went with CP forged pistons and rods supplied by JVAB / John Vanlandingham from the US for my 16V +T engine build.
[2] No. That is to say, not well. Keeping it LH2.4 means at least chipping the EZ-K and ECU and losing a lot of potential. Aftermarket engine management is better. Talk to your dyno shop / tuner to see what EMS they prefer to work with, and get that.
[3] Unworked, unrestored, untouched 16V 532 heads start at around the $500 mark. You may need valves. You will need new valve guides if any valves are bent. You will also need new lifters if you're doing it properly.
Flow rates for 16V heads start where 531 heads with a lot of porting, valve work, etc finish up.
You can do better than the factory inlet manifold, look at KL Racing's offerings.
Factory B204FT turbo manifolds are hard to find; they were never fitted to any OZ market cars. Beware of the crap on eBay for 16V head headers. Your exhaust choices basically boil down to NA exhaust with J-pipe, a custom tubular header, modifying a crap eBay header so it will fit properly, or making your own. I'll be using a cast FP EVO-type manifold via a very short adapter for my own setup.