I have a regular fiesta mk 7.5 rear bumper and going to buy a st 3 rear bumper I'm just wondering if it will fit and I looked around seems like I would need to invest into a new exhaust? 

Some owners have fitted an ST back box with an adaptor, others have gone for an aftermarket twin pipe exhaust but I'm sure I've seen a couple of threads where owners have simply found a suitable twin exhaust trim and fitted that to the existing exhaust, eg:

https://www.halfords.com/motoring/car-accessories/exterior-styling/ripspeed-twin-round%2Fslant-38---52mm-exhaust-trim-109110.html

ah hopefully I can fit a trim not bothered about the exhaust but would you know if the st 3 rear bumper would fit straight on or should I get someone to fit it for me instead of doing it myself? 

If I remember right, the basic bumpers are the same, all that needs to be changed is to fit the ST upper/lower valence, plus the "spats". Lots of useful info in this thread:

https://www.fordownersclub.com/forums/topic/73775-10-fiesta-ecoboost-mountune-m-r-165-modifications-thread/

A link to rear bumper removal guide:

https://www.fordownersclub.com/forums/topic/92560-fiesta-mk7-rear-bumper-removal-guide/

 

3 hours ago, Lars1865 said:

ah hopefully I can fit a trim not bothered about the exhaust but would you know if the st 3 rear bumper would fit straight on or should I get someone to fit it for me instead of doing it myself? 

You will be bothered about the exhaust when you realise how silly it looks lol. I'm pretty sure the exhaust can rub on the ST diffuser and melt it a bit if you don't get it altered.

I doubt it would melt the diffuser the exhaust is a bit bent like an elaphants ***** 😆

53 minutes ago, Lars1865 said:

I doubt it would melt the diffuser the exhaust is a bit bent like an elaphants ***** 😆

That doesn't mean the hot metal exhaust pipe can't rub on the side of the diffuser. It could be bent 180 degrees and make no difference.

I've seen a boy selling an St exhaust might go pick it up today but would  i need any welding etc done for it or could I just do it myself? 

1 hour ago, Lars1865 said:

would  i need any welding etc done for it or could I just do it myself? 

See page 10 of the first thread I posted the link to. (Initially used a 45 to 55mm adaptor sleeve with clamps and exhaust paste, subsequently had it welded.)