Q1 - what general advices would you give to someone who is starting to think of doing a PhD?
Be careful. be sure you want to do a PhD. Be ready for dedication
Q2 - wouldn't it be too much for me, considering my situation? (2 Master's degree... I know that some people are getting 2 PhDs or 1 MBA + 1 PhD... so I guess it is never too much ahah)
Yes. I think it is too much. In particular in business administration, I think experience is more valued than academic qualifications. Having a Master and looking for HR position could already be marked as "over qualified". If you want academia, you have to start looking for jobs in academia right away after finishing a PhD.
Q3 - while searching for some PhD opportunities, I have seen many PhD in business administration in great US institutions? not sure about other countries...
US is top at business administration. You can google for other countries. UK is also a destination for the field
Q4 - in general, what are the odds of getting a full scholarship? (in a PhD in business administration)
Seeing a great institute and a great position is something, and getting accepted is totally different. Everyone wants to go to US to do a PhD. It is not easy at all. The chances will get higher for non English speaking countries.
Q5 - I am currently 28 years old - but by the time I get accepted (if I eventually get accepted), I will be turning 29... Assuming it will take me 4 years, I will complete my PhD at 33. Isn't a bit unusual? it is just that I know a guy who graduated from his PhD at 25 and another guy who is 28 (my age) with 2 PhDs... doesn't mean much I guess!
This is really funny. I know and you know people at their 60s and they accomplished nothing. So you see 28 years as very old? Why you care about people who got Nobel prize at 20? what does it have to do with you?