I apologize for the length of this response.
I share your concerns with water under the house. In summer of 2009 I added to the back of my house which was built slab-on-grade in other words no basement, just a small crawl space. The addition was 20 x 22 and I put in an 8ft basement. During excavation, there was water at the 7ft level and we dug 9ft to the clay. We filled with 6" crushed stone over all plus inside the footings. During the footing install we had some water and used a transfer pump every 2 days for 15mins to empty. Drainage 4" Big O was installed inside and outside the footings draining to drainable aggregate away from the house. During the fall and winter, sump pump only ran a few times for a bout 3-4 sessions.
In January of 2010 the pump ran much more often and I had a 2nd pump just in case. In the spring thaw it was heavier again but never posed a threat. January 2011, the flow was greater and 2nd pump kicked in and out with primary running full time. Spring 2011, same thing with except that not only primary running full time, but 2nd began running full time but didn't over flow the pit. When 2nd began to run intermittently, I went away for a day and a half and came home to 52" water in my basement with one pump still running. It appears that the flow was too much for the two pumps, floated some wood which kicked off the pump switch at 45" off the floor and shut off the primary pump.
I had to start up the gas transfer pump and get the water down below 12", power off the hydro panel so I could trip the breakers for 2 freezers, dehumidifier and the line to the secondary pump, plug in an extension cord to outside and re-start both pumps. (The freezers in the water did not trip the breaker plus an extension cord was under water at the time of discovery so I couldn't enter the water until I could power off the panel running the pumps.) Flood loss - $12,000. Insurance covered only $9000.
Following that, I purchased a larger pump as primary(3800gph) to run with a secondary at 2250gph. I then had a contractor dig a trench on the low side and installed a catch basin and 3 - 100ft Big O pipes, two solid and one perforated. I perforated the solids along the 20ft part of the basement so all 3 would gather water and send it away.
Not trusting the secondary pump for much longer, I purchased another larger pump, this time a 4200gph and installed it as primary with 3800gph as secondary in Spring 2012. January and Spring 2012 and 2013 have been okay with the exception of 2013 spring, we had a 12min power failure and flooded 2" in the basement before I could get the gas pump primed and running. I have since acquired and set up a manual generator back up that will take about 4 minutes to operation(as long as I am home).
So, in summary, the drain lines are adequate all year round except the January and Spring thaws. The pumps are only activated for the excess and January 2014 the secondary pump never came on. Just when I thought I was getting ahead of Mother Nature, she keeps me on my toes. I live at the base of an escarpment that rises 400ft behind me. This year was extremely heavy for snow and as I write this, I am hoping that the peak run-off is behind me. My primary pump is currently back to running 4.5 minutes and off for 1.5 minutes as the flow has settled down. My pump started running 09Apr14 with the primary(4200gph) running full time from 10Apr through today 13Apr at 7am. The cost for the 3 drain lines was $2500 a premium for where I live in the bush with trees all around. So, depending on your area you may or may not experience as high a cost. Another time I would rent a bobcat to do the job myself and if it took me a week, my cost would be $1400 at current rates.
All the best with yours. Feel free to respond if you have any further questions.