Headers can be a real pain in the neck.
When you are looking to buy and compare, remember that the thicker the flange plate the better chance you have of getting a good seal.
Headers are known (all brands) for springing leaks at both the upper head to header gasket and the collector.
Unless you want to spend your spare time fixing leaks, or having to listen to the tick tick tick of a blown gasket, go ahead and spend the money on the best quality you can buy.
Second, make sure you invest in high quality header bolts with locking retainers. Trial fit all of your header bolts outside of the car. Don't be surprised if you have to ding or dent the header pipe, and or cut one or two of the header bolts down shorter.
Check with the vendors here for the parts.
I know Summit has the locking header bolts under
State 8 # STG-8911 $31.95
I have used these in the past and been very happy with the long term results vs. just header bolts and washers.
Finally, good quality gaskets. Felpro is excellent. You want a good quality collector seal. Again, Summit has some "dead soft" copper gaskets that work great. Paper gaskets at the collectors will eventually blow out.
Yes, any quality muffler shop can cut your stock pipe and weld the collector mount to your header collector.
You will have to pre-cut your stock exhaust as close to the header collector as possible to give the exhaust shop all the pipe they need to connect. Install the headers and either trailer or drive real careful to the local shop to have them complete the under car welding.
I would also suggest spending the extra money to get ceramic coated or get your headers ceramic coated. It makes a difference now, and long term.
Hope I helped.
