The difference between Core i3 and Core i5 is something called Turbo Boost. It is an automatic overclock that increases the base speed by as much as 700MHz depending on the model. Some i5 CPUs may have lower max Turbo Boost speeds and that max speed is only when 1 core is being used. If both cores in the i5 CPU is used then it will only be a 500MHz increase (depending on the model). That is assuming the CPU temperatures is not too high.
The more current the generation the better. Haswell is the current generation any Core i3 or i5 CPU model beginning with "4" will be Haswell; for example the Core i5-4330m.
I generally prefer Lenovo (formerly IBM) ThinkPad T series when it comes to a pure business oriented laptop. Below is the link to the various T series models. You will see several negative comments regarding the T440p from "hardcore" Thinkpad owners because of the change to the keyboard which now uses the chicklet / "island style" keyboard that are one every single laptop out there as opposed to the more "traditional" style keyboard where the keys are relatively large and close to each other. Personally, I like the traditional style keyboard since I make less typos using them. Another gripe is the buttonless mouse pad. Again, this is the trend, most laptops no longer have buttons on the mouse pad. Not a big deal for me since I use a mouse.
http

/shop.lenovo.com/us/en/laptops/thinkpad/t-series/
The ThinkPad L series is a more economical version in the ThinkPad family:
http

/shop.lenovo.com/us/en/laptops/thinkpad/l-series/