New window opening / lintel

Shouldn't be a problem for a builder as long as you get a new lintel

The correct lintel size will be selected in accordance with the width/span of the new window overall and the load from above. Just approach a builder first to take all the dimensions. No need for an engineer. They will just want to put a steel in everything to be extra safe. You will probably do fine with a reinforced concrete lintel which is usually cheaper too. Builder first as they will give you all the possible options.

It would very much depend on load from above and head height if near eaves. Generally a builder that knows which lintels are available and has extensive experience in installation, would be able to give you guidance here. However the span is borderline for a steel and definite no no for concrete to span that width. It would come down to loading and if for instance it has to support upper floor and walling there's a high chance even reinforced lintels could twist/buckle in this scenario.

As your making the appeture wider it all depends on what weight is above. It would be good if you got a structural engineer to make sure you are inserting the correct Lintel or r s j after all necessary propping has been put in place. Hope this helps