At minimum you're going to need airbags to help the soft springs. Whatever you do, none of it will increase the trucks ratings that you'll need to carry that weight. No 2500 has 4K payload. You'll also be exceeding the axle and likely tire ratings too.
If your looking to buy, why not just get the 3500 and not worry about it? The price difference is minimal. I just factory built identically equipped 2500 and 3500 trucks and the difference was less than $300. No point in starting under equipped.