2.5 or 2.0?
Tax is about 300 per year dearer (pre '08) for the 2.5.
Insurance is the same as is the mpg (well for the STIs).
Out of the box the 2.0 is a stronger engine (read hardier and more reliable), but less power (headline number similar, but way more torque from low revs with the 2.5). The 2.0 is a bit more freer to rev, but that is needed as your power band is 4-7k rpm. A Spec C obviously has better low end torque for a 2.0 due to the twin scroll setup and has better suspension bits too, but the 2.5 still a better torque spread. The Spec C would command a significant premium.
Out of the box the 2.5 can suffer issues, but usually issues are only surfacing after a custom map with more boost. Any standard 2.5 Hawk with oem boost levels should be ok (if it was going to fail it would have by now). Powerband is roughly from 2700-5800 rpm (after that power tails off and might as well grab another gear). Any 2.5 with increased boost will (eventually) need upgraded head studs at a minimum to prevent the heads from lifting. When doing that job might as well throw in a set of upgraded pistons as the standard ones have known ring-land issues and most of the labour is already spent by that time to do the head studs anyway. After that the 2.5 is probably good for a very reliable 400-450bhp depending on supporting mods. €3500 would build you a reliable engine able to withstand 450bhp/600Nm, although at that money you'd only see about 360bhp/540Nm levels of power as to go beyond that you'll need other injectors and turbo etc when running RON95.
The JDM Hawkeyes are indeed demanding a premium. Probably enough that it would be close to a 2.5 with a budget to do the needed engine work... unless you get really lucky and find a clean enough 2.0 JDM for small money. As per above the Spec C or S204 are more money again. Bringing a clean one in from Japan is very costly at the moment due to exchange rates and VRT situation... so chances are anything brought in from Japan at a competitive price will not be as clean.
The above gives you an idea of rev range and costs etc... So the question for your friend is how he intends to use it.
On the road: 2.5 is king I would say because of the rev range and low end torque. This is based on my own experience with my 2.5 Hawkeye and having owned and driven 2.0s too.
On the track: 2.0 a good option as you'll be spending more time in a higher rev range anyway, but a build 2.5 engine will still be a formidable weapon and maybe more forgiving for a novice driver due to the wider torque spread.