Zetecs are pretty cheap and commonplace. Plenty of Mondeos always being scrapped! Just have a look on Ebay for a start. Middle of the road weight wise (about 120 kg) and go well if you junk the factory induction system, which you will if you want a bonnet on your car. 165 BHP from a 2.0 on a set of old bike throttle bodies (more on the later black top engine which had a better head) before you make any internal modifications. Pretty much bomb proof.
VX C20XE as above but a little more power, and a little more ultimate tuning potential. Finding good ones is getting less easy now though.
4AGE is a nice engine. Compact, light, unbreakable, revs to the heavens.

The Ford Duratec I4 is starting to look a very nice choice now, and starting to become available 2nd hand. Considerably lighter than a Zetec (90kg - alloy block), more cubes than a 4AGE. Good and growing support in the aftermarket tuning arena. Mate of mine has a breathed-on 2.3 in a Caterham. 260BHP. Very, very nice package,

although it didn't come cheap. 2.0 Ultimately has more tuning potential than the 2.3 due to shorter stroke but 2.3 gives more bang for the buck as standard.
Even our old friend the Rover K series isn't a bad choice. Lightest of the bunch and the head gasket issues that plagued production variants aren't an issue if you build them right. I have many mates who run these in Caterhams, including some with a reliable 240 BHP @ 8500 RPM.
VW/Audi 20v Turbo....

Keep thinking about it. One day...
Unfortunately, if you want more than 4 cylinders engines get much less optimal IMHO. Forget the Ford Essex / Cologne / Cosworth BOA. Totally outclassed these days and a real boat anchor.
Rover V8 - plenty of tuning support but it is seriously long in the tooth and will tuning to get significantly better performance than one of the above 4 pots. Not too heavy for a V8, but still much heavier than any of the 4 pots and this will affect the handling of the car. Noise comes as standard though.

X30XE, as said, compact for a V6 due to the narrow Vee. Not especially light. Probably similar weight to a Rover V8 but much more grunt as standard. Plenty of knowledge here, of course, but not popular in the wider kit car world, which makes it attractive in a way IMHO. Ultimate tuning potential? :-/
The Ford Duratec V6 / Jag AJ-6 might be interesting. All alloy so potentially lighter but 60 degree Vee angle and wide cylinder heads might make it a tight fit.
Personally, in a Seven inspired kit I think a tuned / turbocharged 4 pot is the best compromise but each to his own. Others would be within their rights to argue for a big V8 or a bike engine, which all have their own attractions.
Fuel injection of some kind is a no-brainer these days, and required to get through SVA anyway if you have a post-Aug '95 engine. Bolting on a pair of Webers looks like the simple option but they are eye wateringly expensive these days, and finding guys who can tune them
well is not easy.
I had them on my Westfield to start with. Went to a couple of tuners and still wasn't happy, despite a much lightened wallet. Built myself a wideband lambda sensor and started to tune them myself. Got fed up with buying sets of jets, emulsion tubes, chokes, etc.
Put a fuel injection system on it and now I could make changes with no more than a laptop it took me an afternoon and about 30 miles in public roads to get it running better than it ever had on carbs. Range on a tankfull increased from 100-120 miles to 160-170 overnight. I learnt a lot during that process too.
Stelvio is the target, just don't know which year

Kevin