Well, whether you need the extra processing power of an i5 over i3 is really for you to decide. i5 offers higher performance over i3 there's no doubt about that.
The turbo mode will allow the CPU to work faster when it's doing easy tasks (surfing web). When it's doing heavy work it will clock down.
Here's a few links to compare:
http

/ark.intel.com/compare/53434,50072,
http
/www.notebookcheck.net/Mobile-Processors-Benchmarklist.2436.0.html
on average i5 should be about 20-40% faster than i3.
A few things to consider are:
how heavy will your computational load be? how big are you CAD drawings? and how many objects at one time will you be manipulating?
How long do you expect to keep this laptop?
If you can afford to get a better processor it wouldn't hurt, but if money is a big concern for you then you can settle for i3. Waiting 1 second longer to get the same operation done shouldn't be such a big deal. For 70% of your usage the limiting factor will be HDD in all cases.