Find your country, and then the carrier you use...see what bands they use for LTE service. If they do not offer LTE service then it makes no difference what model you get. Also, go to the link at the bottom of that page to see the features list for iOS8 and the countries where available.
After finding the bands that your carrier supports, you can match that information with the band coverage for the different iPhone models. Now, be aware that unlocked iPhone 6/6+ models are not available in the US at this time. The closest you can get to unlocked is a T-Mobile non-contract iPhone from the Apple store (NOT from the T-Mobile store) or the Verizon CDMA iPhone. The Verizon iPhone is locked to Verizon on the CDMA radio but unlocked for GSM carrier use.
So, it really does not matter what the band coverage is if you get a carrier locked iPhone you will not be able to use it in your country. Your cousin needs to be sure and get either a T-Mobile non-contract iPhone 6 ($649 in the US) or a Verizon iPhone - and those will only work on LTE if you have the same band coverage.