How much stress could the engine take?

It depends on a lot of things. Mostly a good level layout and well made art.

Be reasonable with the models. I don't really know what kind of game it is, but the same principles apply. Don't use more triangles or vertices than you have to. Modern GPU's can handle millions of verts so the ceiling is fairly high, but you should favor normal mapping and parallax mapping 99% of the time over adding more details to your geometry.

Draw calls are probably the biggest bottle neck you will run into on the graphics side. This makes Texturing and materials are critical. Unity will batch together "static" meshes together that share the same material. This let's you draw lots of objects with just a single draw call such as a forest. Texture atlases are important for the same reason. Have multiple models share the same texture and they will get batched together.

This almost goes without saying, but don't use bigger textures than you have to. It wastes processing time and takes up memory. Most character models don't need a greater than a 1024 x 1024. If you won't be looking at something up close then use a smaller map. For a realtime strategy, you could probably get a way with 256x256 on your units because they are so small on the screen. You should keep mip-mapping on most of the time. The only exception is with a 2D game you can turn it off because mip-maps take up extra space. AND USE TEXTURE COMPRESSION ON EVERYTHING (except for the gui.)

Then use good level design. You can take advantage of Occlusion Culling to greatly increase performance. Also, consider asynchronously loading levels in the background if your levels are that big. You will start losing precision as you get further away from the origin and it will create excessively long load times.

Scripting is... well scripting. Write good code and you should be fine. Cache variables (especially references), use static typing, avoid un-necessary lookups, and so forth. Most programming principles carry over.

It's hard to give benchmarks because it depends so much on the game designer. Cartoon Network's MMO Fusion Fall used Unity 2.x and 3.x is faster than that. It is pretty comparable to shiva3d and (I shiver to say this) Torque3D.