Vince, welcome aboard! I don't have all the cars accounted for (although that is the goal), but you are definitely the first I know to be taking an M10 to New Zealand...You will definitely enjoy the car and the car will definitely garner some attention.
That range for 1/4 mile times is probably pretty accurate...I say that because the 15s are either from guys that can't drive, or more likely higher altitude or bad air. I've had a few tell me they click off 13s at sea level (Houston, New Jersey, etc). I'm in Amarillo, Texas and our track isn't exactly the greatest (I spin more at the track than on the street, lol) but I was babying the car at the time and was running 14s (the motor was right around 500 miles etc...super soft launch, and going from a stand still). I have been over to Noble, which has a much better track and air, but go figure, the show I was at was during a record high temp span...the show was at night and the asphalt was 109 right at sunset. I think I ran mid 14s there, but also wasn't cutting good lights. Both of those runs were all box stock from the factory with no additional performance mods.
I have since done a few things more to the car (catback, tuner, etc) and we had a dyno day in September. I put out 348 HP, 372 TQ. I wasn't the one behind the wheel and since I was filming for the club, I was the last to go (heat of the day).
I'm not real sure the catbacks add much, but since you are an Auto, and based on your past lineage, you will want to go that route to add some bark to the car...you will not be happy with the lack of an exhaust note. The cars don't sound bad full throttle with the stock exhaust, but at an idle you won't know the car is running. Plus, the factory auto exhaust is nuts...huge suitcase muffler in the middle with two giant resonators in the rear...they actually bend up and recess behind the rear side markers...Even if your gain for power is minimal, shedding the weight and the sound is worth it. As for brands, it comes down to personal taste etc. If there are any other 5.7s down there (ie Rams etc) you might listen to them...the sound will be similar.
In terms of the motor, it depends what your goals are. Magnuson is a proven performer and looks quite well imo. A guy in the club just put one on his 5.7 B5 and did it in his garage over the course of a weekend (working very slow). If you would be content with that I would leave the internals as is...
If you want to do a full build, I'd just get a stroker with a bottom end that can handle more boost. The prices for built motors are crazy and sadly, the parts are super expensive as well...so even if you have a great machinist, the cost adds up fast. The late models definitely aren't as budget friendly as the LA/B/RB for internals.
You could probably pace yourself...Start researching sound clips on YouTube or locally...find an exhaust note you like and order it. Get it and a tune installed and then decide if you want to pull the trigger on the supercharger or do a more intensive build.
Several have started doing a 392 cam swap fwiw...
I think the autos would definitely benefit from more rear gear for drag racing...Only tradeoff will be your MPG if that is a concern. You wouldn't have to go too crazy...even something in the 3.55-3.73 range would make you pull any similar cars pretty easy. I can't remember the exact ratios for the new diffs, I'm pretty old school myself.
Obviously, you are getting a special car. If you decide to put a shaker on it, you'd want to keep the hood...My thoughts are if you want to "upgrade" it, just pull the stock parts and set them aside. You could even do that with the factory 5.7 if you wind up shipping in a built stroker. I've done a lot of stuff to my car, but I did it all with the realization if I ever wanted to go back to "stock" I could do so, and with parts in mint condition.
I'm also not sure how big into stereos you are, but if your car has the 130 4 Speaker like mine did it flat out sucks. I've griped at Chrysler about it. If you got the 230 6 Speaker etc you should be fine, unless you are more of an audiophile. This is one of those deals that I replaced, kept the factory stuff, but can never see myself putting it back in...I kept the factory exhaust as well, but it'll be a cold day in hell before it goes up under my M10 again.
If you think along those lines, just be smart, keep the stuff you replace, and in the event you find the car is a hot ticket item years down the road you will have all the stuff
One last note, if the previous owner hasn't mentioned the owner's kit etc, please hit them up about it. Some of the stuff will have their name on it, but you'd still want it and they would have no use for it without the car.
Definitely keep us posted on your M10 and what all you wind up doing...As well as the brand-x slaughtering you do. I'm kind of bummed to hear there are crud tons of mustangs down that way too...I thought it was an American problem, lol
I have several sound clips up on LoneStarMopars and AmarilloAreaMopars YouTube Channels if it helps you out.