It's not hard, but it is time consuming.
Any exhaust shop can install one for 2-5 hours of labor (depending on the shop). Most exhaust shops charge $70-90 per hour.
If the previous owner took the cats out than, they likely installed an aftermarket downpipe, which are more likely to crack than the stock one.
Used ones can be found both here on Clubwrx and NASIOC
Cheap downpipes can go for as little as $120-150 on eBay. Most cost between $200 and $450. Really Expensive ones that include the Catalytic converter can be as much as $600
If just replacing a broken part then this will not effect gas mileage, and have very little effect on weight.
Something that will need to be addressed is if the check engine light is on. In that case please read this thread.
Yes, you need an EM for engine mods
Also brows though the "FAQ" link in my signature.