High spoke count wheels will help. Properly tensioned and stress relieved wheels are more important than spoke count but when you combine all three attributes you should have wheels that last without trouble. I started with a Schwinn Broadway that cost me $200 so you can go that route however I had to within the first year replace the wheels and that cost me more than the bike. Given that you wish to ride paved and unpaved trails I'd suggest a hardtail mountain bike with a fork that can be locked out. Suspension will rob you of power but since you are after fitness and weight loss putting power to the ground might not be important to you. Resist the temptation to increase your eating to make up for your exercise. You can gain health and lose weight and it is worth doing. FWIW I started at 270ish, gained to 289 in first year of riding and now am at 214.4 and shrinking.