Since the gearing of the tranny is probably going to be a huge factor in mileage for that CRX, you may want to consider swapping in a b18a tranny. The b18a is a lower revving motor, unlike the be b16a, so it should keep your revs down much better while cruising on the highway.
To this same point, you may also want to look into swapping the "final gear" (think of it as the rear gear in a rear wheel drive car, the differential gear), since that would allow you to keep the b16a tranny, but you could still adjust the RPMs in all gears, to make you get better mileage.
The CRX is a great car to start with for MPG since it does very well in the aero aspect, and is a somewhat light car. Though it is nice, you may want to look for a CRX that doesn't have the DOHC motor and the glass roof. If you can find one with a SOHC and without the glass in the roof, it will be a lot lighter, and should make getting more MPG much easier.
There could be one BIG problem with using a B18A transmission here: they might not be available abroad. In MOST of the world, the 1990-1993 Integra used the B16A, just like the CRX SIR. I don't believe the B18A was used anywhere but the US and Canada. So coming across a B18A transmission could be quite difficult and/or expensive in New Zealand.
As for swapping over the final drive, this will actually make matters WORSE. Although the overall gearing is shorter in the Y1 transmission used in the SIR than in the B18A S1/A1 (B16A Integras used a different S1 transmission), the Y1 actually has a TALLER final drive than the B18A S1/A1 (4.266:1 vs 4.40:1). So this is NOT something you want to do to save gas, or, for that matter, drive on the freeway. On the other hand, some people are have been known to swap the B18A S1/A1 fifth gear into VTEC transmissions (like the Y1 and B16A S1). This gives you good ratios in gears 1-4 for acceleration, while giving the benefit of a taller fifth for better fuel economy on the freeway. And since the Y1 actually has a taller final drive, overall gearing will actually be TALLER on a Y1 with a B18A fifth than on a straight B18A A1/S1. On the other hand, this setup is good ONLY if you drive alot on the freeway. If you drive alot in the city or on mountain roads, you will likely be forced to stay in fourth rather than shifting up to a too-tall fifth (which would hurt fuel economy).
Of course, getting the parts could be a problem. But if worse comes to worse, you could probabl just phone order the parts from a US Acura dealership. Some actually WILL ship parts.
As for whether to buy the car, this all depends on YOU. If all you care about is fuel economy, then there are better choices if you want a CRX. Obviously, the best choice would be a CRX HF (or whatever equivalent is available in New Zealand) with a D15Z1 swap (1992-1995 VX). And if performance is what you want, you are better off with a lighter CRX (like the HF) that has a B16A or, better yet, a B18C swap. This combo would probably also net better mileage than the SIR due to the lighter weight AND the possibility of using newer (1992+) electronics, which deliver better fuel economy. My 1988 CRX DX with Integra Type-R motor, for instance, delivers about 33MPG on the freeway - higher than the SIR because of lighter weight, higher compression (11:1 vs 10.2:1), and newer electronics (I'm actually running the proper P73 ECU). It also traps at 102MPH in the quarter mile, and could probably run 13s with someone behind the wheel who can actually launch the thing right. The SIR will not do ANY of these things.
On the other hand, a true SIR is a modern classic of sorts and rather rare. It has the potential to increase in value in the future, which cannot be said about a lesser CRX or one with a swap. So you might buy it for this reason alone. On the other hand, a second car might not be a bad idea if you want better mileage. After all, you probably want to keep that SIR in tip-top shape.
I'm not actually thinking of getting the car - mainly because, it is beyond my budget for the next while - by the time I have the money, that one would be gone.
Also I probably wouldn't be the best owner - I would keep it in good shape, but it would have no spoiler, moon discs, wheel skirts, grille block, undertray etc very soon!
One of the attractions of the CRX is that it is very light, and quite aerodynamic, so would be a great car to hypermile (should surpass my original Bluebird nicely!). It would also be a good car to put an electric motor into!