Traveling Overseas Lens 3 & 1/2 Months

[msenior]

Hey guys
Been a while since I have posted on here. Hope everyone is doing well.

I am heading overseas for 3 & 1/2 months in 5 weeks and I am having to purchase a new lens before I go.

I did have a Canon EF-S 17-55mm f/2.8 IS USM but it broke on me back just before Christmas.
I was going to get it fixed but $480 to do so didn't seem wise when it is about $900 to replace.
A mate wanted to try and fix it himself (engineer) so he bought it from me for $250.

My dilemma is that I possibly might want to upgrade to a 5dmk3 (from my current 7D) sometime in the next 12 - 18 months.
Do I buy another 17-55 or do I go to a 24-105L instead so I don't have to buy another lens when getting the 5D anyway.

The issue then is what is going to serve me best while I am away. I have been shooting with my 17-55 since I bought my cam in 2009.
I love the 2.8 for low light and going to the 24-105L I won't be as wide and only at f/4. Will I miss not being so wide if I go that way and I think I will miss 2.8.

I have read some other forums today and they have suggested a 15-85 which is wider obviously than the 17-55 and more focal length. It also is as about as sharp as the 24-105L. (So they say)

I am kinda of torn. My brother has been traveling around Asia for the past 12 months and he took a 18-135 and he said it was fine for what he wanted. I have borrowed it and used it but not sure about it seeing as it isn't a set f stop.

Anyone able to give me some advice on what way would be best.
Even if another lens/brand would suite better I am open to suggestions.

Cheers
Michael