we are building a new MVC 3 application to support our business, timeline is aggressive and plenty of features to implement. I would like to understand if supporting also IE7 instead of only IE8, IE9 and Chrome would slow down development or if in the end is only about some minor adjustments in the css files.
provided that all javascript / DOM selections/manipulations will be done with JQuery can we imagine all the client side code will be fully working on IE7 and IE8/9 or do you think supporting IE7 as well would become a major effort and not only minor styling exercise?
Thanks!

If you are lucky enough to have to only rely on the jQuery dependency then you should find that supporting IE7 is pretty simple if you are already supporting IE8. You will not find this to be the case with other opensource js projects (OpenLayers for example is not fun to work with even though it claims cross browser compatibility).
What I do suggest is that you build generic content and css templates beforehand and ensure that they work in IE7. You should find that you will not have any other issues. I wouldn't underestimate the overhead fixing css can cause. We did have to rework our entire css at one one point as these so-called ie7 fixes started to become unmaintainable (incremental hackery is likely to become unwieldy).
It is all dependent on what you are doing exactly though, but in the average case should be quite straightforward. Dealing with browser performance and memory leaks in web applications is a whole different ballgame.
My personal preference would be to just support what you must. I am happy to ditch ie6/7 wherever possible, but I appreciate this isn't always practical if you want to get paid at the end of the day.