You are looking around £1500-2k mark to do it yourself, a lot more if getting someone else to do it. Mtec (ME221 boys) do it for a great price, but it may be a basic set up, though this will more than meet your wishes for 40bhp more. I’m currently doing it to my mk1.
You should easily see about 200bhp with the following -
Exhaust manifold and downpipe, Black Cat Motorsports or G19, around £500
Subaru impreza td04 turbo, ebay around £80-120 usually (know what you’re looking for in a good turbo tho)
Intercooler set up. Fast5 sells them as a kit on ebay for about £300, but I’ve bought individual bits as plan to make a different pipe run
RX8 yellow injectors, ebay about £100
You need to drill and tap your sump for the turbo oil drain
boost controller unless you want to run wastegate pressure, £40 (MAC valve)
Stand alone ecu, £500 I’ve gone for the ME221, its a pnp unit and very easy to play with. Other options are Megasquirt.
Free flowing exhaust at least 2.25" dia, £200
Variable tps, GSF, BMW e36 unit about £45
Air temp sensor, GM unit about £10
There may be some parts I’ve missed as this is all off the top of my head!
G19 can do them and is in Ipswich, if that helps you, and seem to quote around the £2k mark for most of the above fitted. You supply turbo and injectors.
That’s how I initially thought, but things progressed and then it got to the point of no return! Life is too short! Go for it!
The good thing is, 2nd hand turbo parts hold their value, so you could do it, then when you wanted to sell, strip it down and return it to NA spec to sell, and sell the used turbo parts! The mx5 is fun with 130bhp, but just too slow! Now imagine it with nearly twice the power! Then its fun!