Hi welcome, feel free to get stuck in and get some pics up [emoji16]
Lambda sensors are a weak point, if your handy with spanner it can be changed in 10mins.
If you buy a cheap odb2 scanner and an app for your phone you should be able to determine if it's the front or the rear,
As for decatting you'll need a remap as it will bring the management light on permanently, also could cause over boost issues.
How much of an increase in horsepower are you after ? As a front mount isn't really necessary on new age cars unless you want to go above 350bhp
This will also require mapping in, if you were to go down the remap route you'd need to budget for an uprated fuel pump to avoid running lean under increased boost.