Hi, and welcome to MKIV ownership, you have many years of fun and tinkering ahead of you!
Firstly, 15-inch wheels are a great first step. They improve the feel of the car on the road, enable you to use Avon CR6ZZ tyres (superb), and look great on the car.
Regarding engine output, I'd not rely too heavily on what the factory data sheet says as the same sheet was used by the factory irrespective of 302-block engine used, headers fitted, exhaust manifolds used etc. If you really want to know its current output, put it on a rolling road. Just don't get your hopes up, as it may output less than 225!
However, based on your intended use of the car (wife, long distance rallies, MPG), you may want to use it for 2-3 years before changing the engine. In my old MKIV and running the factory fitted engine I would get around 22mpg combined cycle on a trip across France and back and a little less around the Surrey countryside on weekends. Once I'd changed to a 347 stroker, consumption went down to around 15mpg on a lucky day and into single digits when pushed harder / Surrey weekend blasts.
Decide what you want a faster engine for: Yes, it's louder and acceleration is quicker, but the aerodynamics of cobra's are not great hence there won't be much difference to usable top-end speed (plus your wife won't enjoy it!).
By all means consider a new engine, but beware of it's impact to your pleasure in the car. If your wife tires of the noise and you tire of an empty wallet each time you take it out, you'll become less inclined to do long distances and hence may actually spend less time in the car than if it's left 'as is'. Also, apart from the cost of the engine build itself, factor in another £6-7 for engine installation (it cost more than I was expecting).
The one thing you've not mentioned is suspension. Have you got front and/or rear anti-sway bars fitted? And have you had the overall suspension set-up look at? Getting the suspension etc tuned will deliver you a car that's far easier to drive, gives you more feedback, and actually allows you to go that bit faster using your existing engine. If you do have it done, do it on the wheels that you intend to use long term (i.e. don't set it up on 16" wheels and then fit 15" wheels, put the 15's on first). It will only cost you around £3-400 to do (assuming that you already have anti-sway bars) and is a very worthwhile investment.
Overall, the MKIV is a superb car and you'll have tonnes of fun whatever you do. Each car becomes as individual as its owner which is great, but take your time. Get to know it 'as is' first as it will save you money in the longer term and make you happier.