ok. to TC TS is abit tricky. Coz im not sure are they using the stock wrx turbo manifold or custom made a whole new one. If its to custom made, then its going to cost u quite abit as boxer engine turbo manifold is not easy to fabricate. Trust me. done once. but if it able to use stock wrx turbo manifold (which i doubt) then you do save alot.
the tricks is here. For boxer engine, there is an additional piping called up pipe. So you will have turbo manifold, up pipe and down pipe.
As for purchasing parts seperately, hm... wat your fren say is correct but you have to factor this. If your parts faulty? If your parts cant be install as it is? What if alot of customization needed? So at the end of the day, you may not really save cost as its very normal those mech will find fault on the thing you buy seperately and also they will install just like that even somehting is not right without informing you. Thn whn problem come, they will say, all this thing is you bring to me one and i just help you to install only.
moreover, not many plp experinces in boxer engine not to mentioned TC-ing it. Dun come tell me Viswork or Speedworks. u will cry big time.
So my advise is, get the complete turbokit from the forum. nego with the seller or wait for good deal. I DID saw some good deals like past 2-3 months back. On those kit, you may find the price may abit high but it save you thru all the hassle and time. That is really more on BOLT ON. If you're saying converntional 4-inline turbo kit, then i can tell you head to toe. but boxer its not that easy as it seems. I did some homework on it as i do have a thought of getting TS and bolt on but drop the idea subsequently.
yours already in illegal way so i dun think its wise to do FMIC as you get more unwanted attention. go for original top mount