Leganoth, if you are looking to do a Templar kit then no plate for the most part; you might could get away with plate defenses on the lower arms and legs, the knees perhaps, but definitely no breastplate or cuirass. Having said that, if you went with one of the other, longer-lived orders, then you could go plate to a certain degree. Sir Nathan can tell you all you'd want to know about the Hospitalier and Teutonic Knights, myself and a few others can go at length regarding the Templars were you so inclined to read/listen.
First things first though...what do you ultimately wish to do with this kit? IF it is just for show, then 18 gauge is perfectly acceptable...it will have the look without the requisite weight. The mercenary harness on medieval collectibles is a good example.
Mercenary's Tailor is a separate entity; now Allan, the proprietor, makes all of his armor by hand to your measurements...when it comes to armor, it has been my experience that buying off the rack will only lead to disappointment as it wasn't built for you specifically, but for a range of sizes. Armor made to fit you is much more comfortable to wear, is usually better made than the off the shelf stuff you see on line and will last longer and withstand more abuse. Not to mention is pricing is very friendly.