adding tech/constructs into a game that has none to begin with can be challenging as well as awkward since its add a new dimension that wasnt there to begin with. However it sounds like you're completely on board with the idea(i assume), and you have an idea about how you want it to "look" as well as how to get it. For the past year i've been playing in a campaign as a construct crafter, and have taken a look into the 3p Tinker class.
Theres multiple ways to incorporate it, but thats not really worth addressing yet. Keep in mind balance. I dont know how familiar you are with construct crafting rules, and since this guy seems to be wanting to go that direction, theres some decision to be made as a GM. Theres several levels when it comes to making them that can really impact the balance of using something like that.
That being said, if when you run into the "automatons" in the ruins, and he starts replicating them, then you can get real personal with what your allowing him to have access to(good thing as a GM).
In our game we dont use 3p material. That being said, i have looked at most 3p crafting options that they give, and a lot of it is just ironed out/re-worded for QOL reasons that crafting imposes on people who try to utilize how good crafting really is. So if you allow these classes, more power to you. I havnt looked in-depth, as in number crunching, character builds, and weird class bonuses they give you in too much detail. But note when it comes down to it crafting is very strict and has great guidelines that are laid out to make everything disectable and easier to digest.
Now you said he wanted to use constructs. Is it because of this specific class(Tinker?), or is it because he wants to use constructs and found this class to serve his purpose. Either way, from what ive seen about Tinker, as well as other construct oriented class options out there, they look to good to be true on several levels. Theres certain aspects the creaters, i feel, put into the game to limit the way crafting is supposed to work in an adventuring scenario. Whether it be something simple like money, more janky like crafting time while traveling, and more complicated like CR distribution and severe crafting requirements. These are all put in to balance the fact that if all goes smoothly and according to plan, you have an end product that will be able to go 12 rounds in the ring with a martial PC.
If he wants to use constructs, theres ways to simplify all this crap that i had to learn. First off, just go off the basic bestiary construct list and construction requirements. Dont even touch custom constructs unless you want to achieve something specific, unless you want to do quite a bit of homework(which i wouldnt). These guidelines will give you solid on paper numbers to live by. Dont worry about modifications either. For the most part they're just basic ability, stat, or armor/wep modifications, but can get more complex. Templates are also an un-necessary, yet extremely fun, option. I could go on but i wont for fear of dragging on too long.
If this guy wants to use constructs, and as a GM you're cool with the idea then have fun with it. As a GM, you can do anything you want, so do that. Im sorry i couldnt be more helpful on the class itself, but constructs is something im (needlessly) versed in. Any questions feel free
Are you referring to interjection games tinker? Anyways, my suggestion is going with a clockwork theme, let whoever this is be the beginning of a new breed, taking his art above and beyond what others do.
Small Ibelieve. What kind of details?
There is no extra cost unless you're making a clockwork autaomata from the bestiary. The feats would all be in the book there are varied movement types but you'd need one of the extra books