I'm more comfortable with it than the others, and my fiance also shoots it very well.
One thing to consider if your fiance will be using the weapon - has she actually loaded the guin with the slide forward or just fired it? Some women (and some men too for that matter) have difficulty with the stiff recoil spring in a Hi-Power and it makes it hard for them to rack the slide if they aren't familiar with semis. This will be even tougher on a .40 Hi-Power. You can overcome it by learning good technique; but its worth taking a look at that issue.
My question is what variant would I be best off with, and what custom touches do you guys recommend for a home defence BHP?
If you are going to get a custom Hi-Power, I highly recommend you visit
http://www.fnhipower.com/
They have write-ups of Kurt Wickmann and Jim Garthwaite's work as well as links to several good Hi-Power smiths.
As for what custom work you should have done, I think that depends on budget and personal taste. I recommend keeping a new gun stock until you have put about 500 rounds through it. This will help you determine what you would like to improve on the gun and it will also give the gun a little wear (which will help the smith determine where it needs to be tightened up, polished, etc.)
The only major thing that almost always needs attention are the sights and a trigger job. The Hi-Power is already highly reliable; but a reliability package only runs about $80. On my Wickmann custom Hi-Power, I've fired 6,000 rounds now with no gun related failures (and only four magazine related failures - all the same mag (Ramline)). One other thing you'll probably want to look at are grips... the stock grips (wood and plastic) aren't bad; but the Spegel grips are great (as are the Navidrex and Uncle Mike's copies of them). The Spegel grips and their copies can also require a little fitting, so it will save you some work to have the gunsmith do it.
However, I'd still wait and shoot the gun stock for a while just in case you decide you want to change something besides those four items after a little experience with the BHP.
Is it worth the extra cash for the HP Tactical?
As to the Practical, the only advanatges it offers are slighty different sights, a Pachmayr grip and a hard chromed frame. Not everyone likes the Pachmayr grips and you already plan to install night-sights so the only real change from a regular MkIII is the hard-chromed frame and a commander style hammer. A gunsmith can do both of those for you if you decide you want them. I'd stick with a plain MkIII for the best value - especially since you plan to do some customization anyway.
What mags do you BHP guys use?
I've had the best luck with Browning factory, Mec-Gar, and Inglis. CDNN offers some 17rd mags from KRD that are a good deal but quality control seems spotty. I've been happy with all of mine but they can require some tweaking or a lengthy break-in period before they are good for self-defense use.
Here is a
Browning Hi-Power Magazine FAQ that may help you out.
Lastly can you recommend a place to purchase mags and accessories, and a gunsmith to do good work for a reasonable price?
Again, check out
http://www.fnhipower.com/ for info on the above. I had my Hi-Power customized by Kurt Wickmann and I have been very happy with the work. At the same time, his delivery times are much longer now than they were when I had my gun done - if you can't wait at least 6 months, then you'll probably want to look at other smiths.