I need some input

[98gstaherns]

Ok i finally got my car back from the shop (After 3 weeks) and going to be putting so mods in to it. It is a 98 GS-T. OK here the mods MBC, Boost gauge and air to fuel gauge, 3in turbo back exhaust (2.5-3" RRE Downpipe
Apexi N1 3" catback) These aren't real big mods..But here is my questions.
When i put my exhaust on will it get rid of the O2 sensor? If so will it throw a code to the check engine light? Because I know the downpipe will replace the cat.Then the other is if i put the MBC will this also throw a code too? Ok thanks Guys..
Also does anyone know where to get some tubing for the boost gauge to run?And where to find a T fitting? One last thing is there anyone here who can port a O2 housing for pretty cheap?
thanks i know this is alot but any input will help alot!
-Tyler