That might be a bit too much money. Of course it depends where you are, but I see quite a few older Trek 950's on Craigslist quite often for <$150. This of course, doesn't matter as much if you need or really want that bike now, and that's the going rate for these kind of bikes in your area.
Concerning conversion to STI, it's an expensive thing. The cheapest I could manage was $85 inc. shipping for like new Tiagra shifters off ebay (I'd guess 105 would be around $100 or more for even a good pair used). Add in the cost of the handlebars, the tape, the cables, crankset (that bike probably won't have a 9-speed crank) and oh, don't forget you'll probably need a new front derailleur..... rear derailleur..... crank and chain......etc etc Since you are mentioning 105, I'm assuming you want to go 9-speed, which might mean a new rear hub laced up, or a new rear wheel.
So, here's an optimistic split down of costs (write off those components you have already of course)
stem $30 (put this in as it's likely the original stem won't be the right size angle for you, plus it will probably be size for mtb bars and not road bars as well)
bar tape $10
rear der. $40
front der. $30
new rear wheel/hub/lacing up costs/spokes $100
Also, you might want to get new brakes, but let's assume the ones that come on the bike aren't v-brake types so you can keep those cantis with your brifters, and let's assume you've got those slicks. Lastly, let's assume you have all the tools you need and won't be paying for a shop to remove and install everything.
Keep in mind I think $460 is pretty optimistic -you'll probably pay a little more for some of those parts, let alone if you have to add some more money for a bike shop to do the mechanics or you decide you want some different brakes or saddle. It's amazing, it really does add up -unless you have many of those spare parts hanging around, I'd advise seriously considering saving up a little more and just getting a purpose built tourer, or even consider the non-STI route as that will save considerable money too (you'd have a flat bar, grips, possibly shifters etc already). Having said all that, if you *really* like the bike frame, and you know what you're getting yourself into....... good luck!