Either 15% nitro or gas. Nitro makes a lot of power in a small lightweight package. There is no ignition system to go bad. But is expensive to fly. 15% nitro fuel here is about $30US/gallon.
Gas is normally done by starting with a nitro frame and doing a gas engine conversion on it, like from Helibug using a Zenoah (or similar) two-stroke gas engine. 20-25cc weed whipper engines make excellent gas powerplants for RC helicopters in 600 and 700 size. There are few manufacturers who build turn-key gasser helicopters. Just about all of them build nitro models that can be converted.
Helicopters are harder to learn to fly than multi-rotors. I would suggest taking your Trex 450 to a RC club someplace where there is an experienced pilot who is willing to teach you how to fly it. That is the absolute best way to learn how to set it up, and how to fly it - getting hands-on instruction. The other way is to invest in a simulator and at least get the basics of hovering mastered on the sim. And then move to the real thing.
I have flown mostly modified Align Trex frames, mainly because the parts for them are readily available from just about anywhere, and they are quite economical to build and fly. And they are quite reliable. I also fly a Gaui GX9 with a Stihl chainsaw in it (which has not been as reliable as the Align models). And I am currently building a Velos 880.
I did build a hybrid TriCopter/fixed-wing with a 1 meter wingspan that could do your proposed flight parameters. However it proved to have quite a problem with wind. It’s maximum flight speed is 35 km/hr and it flies for 50 minutes on a single 4S 10A battery. Unlike most multi’s, the power requirement to keep this one airborne goes down in forward flight instead of increasing
I could do more experimenting with it to try to get it so it’s stable in the wind. But the problem I have at present is that the wingtip motors throttle back so far in forward flight that they do not have adequate thrust capacity to keep it stable in the roll axis. It flies beautifully in no-wind conditions at 35 km/hr. About the max wind it can handle and still fly decent is 10-12 km/hr. I fly it with the stock TriCopter code. It could probably be made more stable by building a better wing, using possibly a tractor (vs pusher) prop for forward flight, and add control surfaces to the wing.
So in multi’s something like this is also a possibility - a hybrid. Tri’s are more efficient than quads. Quads are more efficient than hexes. Hexes are more efficient than octos. Helicopters with a single rotor beat them all. Fixed-wings beat helicopters. But with some experimenting a design like this Tri could have VTOL and approach fixed-wing efficiency. If you are the type that likes to experiment and build things, that is worth looking at.
The VTOL code for fixed wings is basically an airplane with a coaxial quadrotor setup strapped to it to make it VTOL. I have no experience with those and am not interested in them because of the complexity and basically dead weight you are carrying in flight. Others here do have experience with them.
Another possibility in multi’s is so-called “tail sitters”. It is a fixed wing airplane that sits on its tail, takes off using TriCopter setup, tilts ahead in flight to where the wing is making lift and uses the TriCopter motors for propulsion and stabilization. Again, I have no experience with those, but I thought I read where the code supports it - tridge possible wrote code for them.
These are all the possibilities I can think of at present. Of all of them in a VTOL platform, the helicopters will lift the most payload. Can easily fly as fast, or faster, than fixed-wings. Are proven and not experimental. Handle wind the best of any other platform. And are relatively simple considering all they can do. There is likely a very good reason in the real world of full-sized aircraft that you see all those helicopters flying around doing everything from air ambulance service to being used for sky cranes. There has not yet been anything invented by man in a VTOL aircraft that can do what they can do.