The investor perk is supposed to add 500 to each merchant (not all merchants have the same amount of gold available), although there are a few instances in which it doesn't work properly due to a bug. Ghorza Gra-Bagol (in Markarth) is once example of this. Also, in case you weren't aware, if you get the master trader perk as well, it automically adds an additional 1000 gold to many (but not all merchants). If you haven't looked at this already, http://www.uesp.net/wiki/Skyrim:Merchants has a full list of merchants and which ones are able to invested in.
I know for sure if you reset the speech perk with the Dawnguard add-on (from the Waking Dreams Black Book) you won't lose the investment(s) you've made. I not completely sure whether or not the legendary reset is the same or not, but I'm pretty sure it doesn't cause you to lose your investment(s) either. Also, see the notes below:
From the UESP wiki:
-Some of the merchants may "forget" that you invested in their shop and lose the option for having extra buying gold. This appears to affect female alchemy merchants much more than male merchants or merchants of general goods stores (e.g. Angeline's Aromatics in Solitude will never remember an investment).
-It is occasionally possible to invest in a merchant multiple times, giving them an exorbitant amount of gold with which to buy. This may be specific to certain merchants—it has so far been observed only with Ulfberth War-Bear and Angeline Morrard.
- Although Ulfberth War-Bear will appear to have a high gold amount after re-investing with him several times, if you invest with his wife Adrianne Avenicci his extra gold will disappear. There is also a chance that when the shops reset after the 48 hour period that his extra gold investment will disappear.