At the moment I don't have an opinion to sway, sorry. We'll need to find out a bit more about you and how/where you ski before we can determine if the ski suits.
But first, if you're getting back into skiing, spending money on properly fitted boots is the best investment you can make. There's a couple of threads running on that same topic at the moment, but a quick summary is to find a good boot fitter in your area and put your trust in them. Boots first, then skis. With properly fitting boots you can ski any ski. With badly fitting boots even the best ski can be hard work (not to mention painful).
Cap and sandwich are just two alternate ways to construct a ski, as you've read. Plus, there are a number of approaches to cap construction (see the attached link below) that deliver the full spectrum of results in the finished ski. Without knowing a bit more about the ski in question I'm just guessing, and I've run out of information on your ski.
In the absence of any further information my one hesitation would be this; assuming the ski was constructed along the same lines as the 76 it may be too little ski for you. Of course, this would only be of concern if the ski is too light and/or flexible for your skiing. If you're "an aggressive skier who puts exaggerated force on their skis", as it says above, then you may want to pass on those skis and look elsewhere.
I've just found this "The 'carbon jacket' construction (of the Head Monster 76) differs from Head’s famed sandwich constructions but it does make for a light and agile pair of skis that are both forgiving and highly accessible". So on the face of it the 76 and the 75 appear to have a slightly different construction. Based on the description in the Levelnine site, it appears your Monster 75 is a bit more ski than the Monster 76, but not as much ski as the sandwich construction of the Monster 78. Again, it depends on your particulars as to whether it would work for you.
In the Monster series Head put out a very well-respected line of skis that stood the test of time over a decade or more (i.e. throughout most of the 'noughties'). The Monster series became the Peak series, and now the Rev series, with various adjustments along the way. Head skis tend to be damp, smooth, often powerful, with a glued-to-the-snow feel, and you generally feel the edge engage with the snow along the full length of the ski. I like them (I have three pair).
Just fossicking around to get a feel for the Monster 76 and I found these threads. As you can see, the 76 (and probably the 75) might not be enough ski for a strong, aggressive and/or heavy skier.
For my $0.02 you can get a more modern, lightly used second hand ski for $400 that will outperform that Monster 75. If you're just getting back into skiing, you're not above average height or weight, and you don't fang around the hill at Mach speeds, then they may work nicely for you. If so, you might like to hack your mate down a bit in price.
Let us know some particulars and we'll see if the 170cm is your preferred length. Another 'general rule' is to size the ski somewhere between your mouth and just above your eyes. Of course, that too is another sweeping generalisation that might not hold true in all circumstances. We'll see.
Best of luck.