The first thing I notice is it is 2-5 players, the first question I have is, "Is there an expansion that adds another player as my gaming group fluctuates between 4 and 6 players."
No, there's not. It would be a good idea for an expansion, since it seems like there are a lot of groups who have just one extra person they need to seat. I'm not sure how easy it would be to rebalance the mechanics, though.
There are a few fan-made variant ideas along these lines.
Also what order do I purchase expansions in and/or which lieutenants do I need (I am very interested in the lieutenants that can be used as player characters as well, as I am not going to be the overlord in my group)
There's no "required" order to buy the expansions in. I think a lot of people just go in order of release date as a default.
As for LTs, it comes down to personal taste really. A lot of people dislike Belthir because his plot deck relies on the Basic I Overlord deck (as opposed to the Basic II deck introduced in Labyrinth of Ruin.) Myself, I like the Farrows.
Serena and Raythen are also interesting.
Also is there a way to get more dice for the game, as I like having my own set, or is there a dice roller for android I can get to use for decent.
I don't know about any apps, but FFG does sell extra dice packs. These dice packs only include the dice from the core game, not the green die that was added in later expansions. However, they seem to be including one or two green dice in every expansion since it's introduction, so you'll probably end up with enough of them over time anyway.