When one sees a sump pump, right away, all kinds of 'scenarios' are conjured up in one's head. It's the same when you hear the word ASBESTOS. To help put fears/concerns in the right place, the 'thing' needs to be 'understood'.
You already know what the purpose is of a sump pump. SO right there, your brain is going crazy. When they do what they're suppose to do, you learn to ignore them. As for your builder's explanation, you might want to get a better explanation from him or from an ASHI-Approved Home Inspector. (I have no conflict-of-interest here; I just 'approve' of their ethics.) Yes, you can have a NEW home inspected! ANd I strongly recommend it. At the same time, I recommend you hire a Real Estate Attorney. These are the ONLY 2 professionals looking after YOUR interests. If you're gonna spend 100's of 1000's of $$$ on a new home, and 2-3 times OVER that over 30 years, spend less than $1,000 to protect your interests. Forget the builder looking after your interests as well as the Realtor (if any.) THey're looking after their OWN interests.
Back to your original question. It's normal to have water in the pit. If you want to see it work, get a couple of buckets of water, and together, dump them into the pit. Do it fast enought so the water rises, yet slow enough so you can see the 'level' of the water at which the pump comes on. WHen the pump comes on, stop pouring, note the height, and wait until the pump turns off, and note the height again. Go ahead and bring buckets w/you and do it a few times. (Hopefully, there's water on-site!) IF you buy the house, I strongly recommend a battery back-up. Power outages during a storm, which is when a pump is most likely to be needed, will be shut off to the pump! GEt a good one. (Maybe someone else can recommend one. ANd make sure it's NOT installed on the Floor!)
As for the outside tiles running into the pit, let the inspector know about this. The only concern I have is the possible build-up of silt. W/O seeing, or knowing, how the tiles were installed, the only way you can know for sure is to see them at your house or at another house being built. The silt that comes in from the outside can easily 'contaminate' the pump. I'm not even sure if this is OK w/code. I know of sump pumps used in OUTDOOR Dry Wells which have tiles routed to them but I've never crossed a situation as you describe. It may be OK but I'd certainly look into how they're designed in your situation (as far as silt prevention). This is where an ASHI Home Inspector will come in handy. See if you can look around the 'other' homes being built. Make sure their tiles lead to an indoor pump. Otherwise, you would be gathering 'mis-information' if they didn't.
If you need more info, post up. In the meantime, watch your Post for other replies even AFTER the looooong weekend. (Some of the regulars are out until Tuesday.) That means that you may see some replies to your post as late as Wednesday or even Thursday. My best to ya and hope this helps.
PS: Those 5-gallon buckets would be real nice to bring along w/you to test out the pump. Again, hopefully, there's water AND electricity at the site!