I'll not comment directly on the eBay ChinaChargers as I don't have any personal experience, but some on this forum have observed or conjectured that the quality of the manifolds on these kits is poor.
Engine management however I will chime in on.
Using an AFC-type device for fuel control is the Wrong Way™
. With the Miata, there are three basic schools of thought on fuel.
There's the old-reliable adjustable FPR. With one of these units, plus perhaps a set of used tan-top injectors from a '94 to '97 Miata (they are *slightly* larger than the '90-'93 blue-tops) you will have sufficient fueling to run 6 to 8 PSI on most turbos. You may find it necessary to replace the fuel pump as well in this situation. Used units can sometimes be found around here, and consider adding a Bipes ACU to perform ignition retard.
Upside: Simple to install.
Downside: Not good above 6 or 8 PSI, doesn't address ignition timing.
Examples: The Bell Engineering model 2025 http://www.bellengineering.net/Pages...uelsystem.html
Then you have the piggybacks- eManage Ultimate and eManage Blue being the popular options. Both will control fuel and timing, giving you boost-referenced ignition retard and the ability to control larger injectors. The EMB (Blue) uses an AFC-like mechanism for injector control, so it is limited in the size of the injectors it can use- 330cc being a commonly accepted maximum for good idle and cruise. The Ultimate (EMU) intercepts and trims injector signals directly, so it can easily run 440cc to 550cc injectors. It will also allow you to remove your airflow meter.
Upside: Not terribly complex to install and tune.
Downside: The most expensive of the options.
Last are the full-replacement ECUs. Megasquirt is king here. Prices for a Megasquirt range from around US$250-$300 if you build it yourself in kit form, to $700 if you buy it fully assembled in true Plug-N-Play for (no wiring required at all).
Upside: The most versatile, and cheaper than a piggyback. MSPNP is also a wiring-free solution.
Downside: The most complex to tune.
You'll want a set of new (used) injectors for any of these. Stock 1.8 tan-tops go for $50 a set all the time. Larger injectors will range anywhere from $75 to $300 a set, depending on how resourceful you are. Used injectors from a non-turbo '89 to '92 RX-7 are a good find- 460cc, and a direct plug-n-play fit into the Miata harness and fuel rail.
A wideband O2 sensor if a good idea if you go with the AFPR, and pretty much a requirement if you do a piggyback or a MS. Plan on $200 to $300 here.