I wouldnt advise getting into Mongoose Traveller or any others if you are going down the T5 route - dont mix and match it gets very complicated. Just stick with one version and you will have a much better time playing the game rather than trying to sort out inconsistenecies between the versions, is my advice for what it is worth.
The only exception to this not mixing and matching would be the various adventures - get some of those they are still great after all these years. Things like CTs Double Adventures, CTs 76 Patrons, Mongooses 760 Patrons (2nd Ed) and Mongooses Spinward Encounters are great relatively version independant supplements that can give you great ideas.
Enjoy Traveller! Its given me many pleasant times over the last 30-odd years and I hope it also is the same for you.
My last peice of advice after all the above has been said is - dont pay too much attention to these forums! Play the game the way you want to. Everyone has their own idea on here about what version they like and how they like to play. Dont pay it too much heed and enjoy your time with Traveller itself.