Since we are being blessed with new character models (Which can be disabled? Yes, no?)
However, I am quite useless on Ordos and Sha since my frame rates dips to 1-10 FPS.
Will the new models cause even more problems in world bosses and even in regular raids?
It's hard to say exactly, many things can influence performance.
Remember that characters aren't (usually) naked inside raids, which means that most of the polygons and textures being drawn and rendered aren't exclusively attributable to player models, but also to armor.
On top of that, raids have their own surrounding geometry, bosses, etc... and all that adds up in the end. Obviously, new (more detailed) models means slightly more processing power is required, but new models aren't the only big change coming to WoD.
We are also applying a squish to anything related to stats; this means that in certain situations, there might be some performance improvements, especially when it comes to addons that process combat data.
More importantly, we are reducing the amount of visual clutter that currently fills every fight where a lot of players are involved.
Less sparkly stuff flying around means better video performance for everyone.
Client performance is something we care about a lot. It doesn’t matter how well-designed an encounter is: you aren’t going to have fun raiding at 8 FPS.
We want everyone to be able to enjoy playing with the new models, and our dev team is working hard making sure that WoW is as resource efficient as possible in order for that to happen.
Shall I start looking for a new computer or should I stick with the one I have?
That's a hard question to answer.
I'd say that you shouldn't be worried about any big drop in performance; things will most likely remain very close to what you are experiencing right now.
So if you're satisfied with what you have right now, then there shouldn't be any need to change anything.