A 250W lamp is comfortably strong enough to provide supplemental light over that area. Only time will tell for sure whether your HPS plus natural light is a good combination for your plants, especially without an accurate measure of the natural light you can provide, but it is in the ballpark. Just provide the best natural light you can, don't keep them too warm, and keep an eye out for any stretching.

A thought on your plan. Do you have the seedlings germinated already? Tree seeds are usually quite slow to germinate and sowing now I would not expect germination until spring, by which time lights might be unnecessary. Generally I would suggest it is much easier to grow them outside, getting a three month head start is barely worth the trouble with something that won't be mature for 10-20 years.

The agrosun metal halide/sodium bulb is avail in 240 watt.