At the end of March the 15th Anniversary celebrations will have finished, but the new April programme will not have started. For this reason the Parks are expected to be a bit quieter than normal. The weather? Hmmm, Northern Europe in March I'm afraid it could be anything from snow through to t-shirt weather. More than likely blustery and a bit showery.
If you get there by about 9.30 I would either........
Queue for rope-drop at Frontierland, walk briskly to get FP for BTM, cut through into Adventureland and be riding Indiana Jones by ten past 10. You then have time to do either Phantom Manor OR possibly POTC before your FP window closes at 11. (Don't go at 10.30 as the FP queue is very long then - leave it at least 20 minutes). If I was without young children I would probably not ride anything in Fantasyland, but I would call in the Castle to see the Dragon underneath. I would then switch over to Discoveryland and FP Buzz, ride Space Mountain and hope your FP time for Buzz was due. If not, I would have an early lunch then ride Buzz, peek at anything else I felt I had to do then cross over to Studios. Crush is the one which will eat up most of your time - could easily be 60 minutes even on a quiet park day. Then I'd do as much of the Studios as I could before it closed, before spending the last hour (or two depending on closing time) mopping up things in the main Park.
OR
I'd start at 9.30 in the Studios, run for Crush, do the rest as quick as I could then go over and get FPs for either Buzz or BTM and queue a long time for which ever I don't FP, then pick up as many of the other rides as I had time for.
As for food, with only one day to see both Parks, I would have counter service only. If you really want a table service meal - see if any of the restaurants are still doing the quick-meal deal at lunchtime.