use an instant read thermometer...lamb is great med rare and that's at 137*F . Plus you'll get carry over cooking. Is it a leg? bone in or out? no bone cooks more quickly. If you want it more well done you can take it to 150F , but it is milder if more rare. I like it both ways.
Yes root veggies around you roast is a plus...potatoes carrots quarter onions etc.
chop up some garlic, parsley, oregano, olive oil, balsamic and salt and pepper.....rub all over lamb and if possible let it marinate for a good while....if not, at least let it get to room temp. before you start cooking it.