I have a 2005 Ford HD 250 6.0 4x4, would like to buy a toy hauler. Is my truck heavy enough to haul one or what do I need to do it to make it ready to haul one?
Depends on you definition of "toy hauler".
Your 2005 F-250 diesel (assuming CrewCab 4x4) has a GVWR of 10,000 pounds. That's probably your limiter. Your wet and loaded truck will probably weigh about 8,000 pounds before you tie onto the trailer, so that's a max hitch weight of about 2,000 pounds before you exceed the GVWR of your truck.
2,000 pounds hitch weight is a tag trailer with 11 percent hitch weight grossing about 18,000 pounds. That's more than your receiver or GCWR is rated for, but if you replace the receiver with a Reese Titan, then you can tow a tag trailer that grosses up to about 15,000 pounds without being overloaded.
So you can look at tag toy haulers with GVWR of up to about 15,000 pounds.
15,000 trailer gross + 8,000 wet and loaded tow vehicle = 23,000 GCWR.
15,000 trailer will have a hitch weight around 12%, or 1,800 pounds when loaded for bear. 8,000 tow vehicle plus 1,800 hitch weight = 9,800 pounds GVW, well within the 10,000 GVWR.
For example, here's a Keystone Raptor tag trailer with a GVWR of 12,500 pounds that would work nicely behind your F-250:
Keystone Raptor | Floorplans
But a fifth-wheel toy hauler has higher hitch weight, and thus will lower the max gross trailer weight you can tow without being overloaded. Typical 5er in that size will have a hitch weight around 18%. 2,000 pounds max hitch weight divided by 18 percent hitch weight = 11,100 pounds max trailer weight.
So if by "toy hauler" you mean a fifth wheel toy hauler, then the max you should look at is one with a GVWR of 11,000 pounds or less. Yes, I suspect somebody makes them, but they're not the monster toy haulers with GVWR of 18,000 to 20,000 pounds you often see around the play areas. And the smallest Raptor fifth wheel toy hauler has a GVWR more than you can tow with your F-250 without being overloaded.
The only way to legally "beef up" your F-250 to haul more hitch weight or tow more trailer weight is to jack it up and run a bigger truck under it.
You can "make do" to fix some overloaded problems, such as air bags to raise the rear end of the truck so your headlights don't point at the stars (and into oncoming driver's eyes). You can run an aftermarket towing tune, such as a DP-Tuner programmer, to give you a bit more power up the mountain pass. But those patches don't change the fact that you're overloaded and need more truck to tow that load.