I have recently been looking into stronger trans options so, I may be able to shed some light. For $1,400 your not going to be able to get anything except a stock rebuild or a used WRX transmission. I have been shopping around and the cheapest STI trans swap I've seen lately was maybe a tad over $3,000. The STI 6-speed trans is considered by most to still be stronger than a built WRX one. If you don't drive like a mad man though a WRX transmission can safely handle stock and up to stage 2 power levels easily. The 2 most important factors in ANY tranny's longevity is driving style and regular maintenance with quality products. You say your hearing noise from the center of your gearbox? Can you be more descriptive of the noise or maybe even a video. It could be numerous things like your clutch, center diff, or more serious internal problems like a mainshaft, shift forks, or the gears themselves. If it is a clutch component or the center diff then your only looking at 300-500 in parts to fix it. I don't know how much new OEM internal parts are but can tell you that for upgraded aftermarket gears your looking at about 2,000-2,600+ in parts alone just for your choice of 1st & 2nd or 3rd & 4th. All four gears upgraded go for around 4,000-5,000+ for parts alone. The weakness in the WRX transmission that is non-existent in the STI is the case design. The WRX case is split into 2 halves. Under high stress it can flex and cause the mainshaft to flex along with it. This causes the gears to not mesh as they should and BAM! You blew a tranny (haha). There is a company called Moore performance that makes "blast plates" which are reinforcing 3/16 steel plates with high grade fasteners that help to combat this case flex. I just ordered some of them tonight actually. Upgraded gears for the WRX come in the form of factory upgrades like the 05-08 Legacy GT gears found here
2005 - 2008 Legacy GT 1st - 4th Gear Set , STI RA gearset found here
STi Standard Ratio Gearset , or aftermarket stronger upgrades like the PPG's found here
PPG 5MT Subaru Straight Cut Synchro Gears and other comparable ones made by other companies such as Albins.
The 6-speed swaps can get downright confusing and for this reason, I won't go into great detail. Some things you need to research and keep in mind for a STI 6-speed swap are axles, gear ratios, brakes, hubs, wheels, clutch, driveshaft, dccd, and rear diff's.
Axles: Some of the STI's use male axles and some use female or "axle stubs" on the end that goes in the tranny. STI axles will not go into a WRX hub. I have heard of making a hybrid axle out of one of each (WRX & STI) but, don't know much about it.
Gear ratios: You have to make sure that your rear diff's ratio and trans are going to work together.
Brakes: You need to make sure your brakes will work with whatever hubs you use and if not find some that will like STI brembos
Hubs: As mentioned above WRX & STI hubs and axles won't fit each other.
Wheels: If you do any swap with STI hubs except the 2004 STI your going to either get new wheels or adapt yours because the bolt patterns are different between STI & WRX (excluding 2004 STI which is the same as a wrx).
Clutch: WRX and STI sometimes use different clutch systems so additional parts may be needed here depending on what the swap comes with.
Driveshaft: Either a STI driveshaft or any automatic impreza driveshaft will be needed because of the difference in lengths between WRX & STI.
DCCD: This is the driver controlled center differential but is completely optional and not a must for the swap.
Rear diff: As mentioned above the ratio of this must be compatible with whatever STI trans you choose. I can tell you that the 07 STI trans swap is considered the easiest because the 3.54 R160 rear diff of the 02-05 WRX works with it. In most other swaps you will need purchase the STI r180 also.