You are about to get "e-raped" for this post so before that happens, here's my constructive $0.02 worth:
You've already answered your question regarding the eBay kits - STOP looking at them.
There is no CHEAP+reliable turbo option unless you can fabricate everything yourself and buy 2nd hand.
Go to https://www.flyinmiata.com/
and look at their turbo kits, buy one of them minus the ECU and get a Megasquirt ECU instead.
The FM kits will have everything you need and is guaranteed to work for longer than a few eBay-kit months (and everything bolts together properly with no missing bits).
shuiend's kit is another good option: MK Turbo Setups
Have a look at the For Sale items here too, many owners upgrade their systems so you can find bargains.
PS: Just had a look at 181437060236. Looks like a bunch of generic parts packaged together (downpipe has no mating flange for your exhaust and none of those intercooler pipes are specific to the miata, you'll need to figure out the combinations yourself and cut them down to size). The only miata-specific piece in that kit is the dodgy china manifold which looks shiny but will crack,
First kit at least has miata-specific intercooler pipes, downpipe will mate to your exhaust and the exhaust manifold looks sturdy - you'll still need an aftermarket ECU and larger injectors though.