Buy rebuilt T5 or repair current one?

Looking for opinions, I just recently purchased a 93 LX 95% stock with 78k original miles. Within the last week I had noticed a slight grinding shifting into 2nd gear upon cold start up, I would shift 2nd gear slowly when cold and by the time the car was up to operating temperature I could shift normally into 2nd again. All other gears shift perfectly fine and smooth. On Friday I drove the car about 40 miles and when downshifting about 25 minutes into the ride to merge onto another highway the trans grinded going into 2nd (even with a rev on the downshift). After this the car grinded into 2nd until I got to my destination and I ended up just skipping 1st-3rd the rest of the way. The next day I adjusted the clutch as it engaged really close to the floor in hopes that would solve the 2nd gear issue, unfortunately it did not and I am guessing I need to replace the 2nd gear synchro.

There is a guy that appears to specialize in mustang parts in my area that has a freshly rebuilt T5 including bellhousing for $400. While I have never torn into a transmission before specifically I am pretty confident in my mechanical abilities that I could get the job done, my thought however is that if I am tearing it apart it seems almost foolish to not do a full rebuild which will end up costing almost 3/4 of what this already rebuilt trans will. Should I spring for the rebuilt tranny and rebuild my current one down the road to keep and/or sell or just fix the 2nd gear issue and not worry about a full rebuild?

Thanks for the input, I definitely want to give the rebuild a try as it is interesting to me anyways and having a spare around isn't such a bad thing. The only thing I am weary of is if the trans does not come with any sort of guarantee which I will have to check on.

Good point on the year of the trans thanks for reminding me of that, definitely don't want to go backwards in that regard. It does seem like going with a rebuilt one for now while I take my time with the first rebuild is the route to take, I'd like to be driving the car and don't want to rush the rebuild so I will check on the year of the $400 T5.

Well instead of buying the rebuilt trans for $400 I took a chance on a used T5 that I picked up for $150...started taking it apart last night and found that the input shaft and counter shaft gears are completely trashed! The rest of it looks fine so now I am looking at rebuilding this trans while still having a functional car in the meantime. Now the questions I have are:

1. Where is a good place to get a new input shaft with gear?
2. Do I have to replace the entire countershaft or can I just replace the gear that is constantly engaged with the input shaft?
3. Anything else I am missing?