I've purchased quite a few obj only products, and I download freebies from Turbosquid all the time. It's not a bad idea to get used to importing obj's because there's a lot of cool props out there for free, and once you get used to the process, it's no big deal.

Advice already given is good.

Make sure you put the obj and textures into a permanent place, so that Poser will be able to find them. Sometimes the vendor does this for you, but not always. I just move the props and textures into one folder together and copy it to the geometries folder of the runtime because I'm too lazy to put the textures in "textures". But that's just me. The scale is usually WAY off, so sometimes you have to scale all the way down to 1% and it's STILL too big. In that case, I scale to about 4%, and then use the x, y, z scale to take it down further (adjust each of them equally).

CTRL-D to get the prop to the ground -- because when you're scaling, the thing may disappear 900 meters into the sky. Sometimes, when you scale a prop, it will end up a million miles from the center. If you have trouble locating the prop after scaling and hitting ctrl-d, go to aux camera and navigate way out. Adjust the "yon" settings on the camera, if it isn't showing you the entire scene.

Once you have the prop scaled how you like it (you may need to load a figure like V4 to see how she looks next to the prop), you'll need to fix all the materials. But usually that's just a case of putting diffuse maps in the diffuse node, specular maps in the specular node, bump in bump and so on.

Once you've done each prop, make a folder in the props tab of your library, center the prop on the scene so your preview shows the item, select the item, and save the prop to that folder. You can parent the props together and save groups of them by using the "select subset" dialog option on the prop save, which will allow you to save everything, or just certain parts of the scene, however you want to do it.

Now you can just load the props from the library like any other.

NOW... you say the main prop is one big welded piece, so it's hard to get the camera in there.

You have Poser Pro 2014, so you can use the grouping tool to cut that prop into pieces. This can be easy as pie or complicated, depending on how the vendor grouped the polys. But learning to use this tool will be like... opening a whole new world... as it will give you lots more things you can do (like making windows and doors that open on props, and making individual walls and floors that move and stuff like that, plus way more if you're imaginative). So practicing with this tool is really worth your time.

This tutorial gives just the very basics of how to select polys and what you can do. It's aimed at grouping for figure creation, but you can ignore that. You just need to learn how to select polys, deselect polys, and create groups. 

http://poser.smithmicro.com/tutorials/Characters_grouping.html

Once you have the right polys selected (red), you can then choose "create prop" and it will create a prop with the name you've given it that is made up only of those red polys. If the vendor has already grouped the prop by either "group" or "material", you may be able to make a good set of scene props by dividing the mesh into those groups. You'll just have to look and see. 

Then, once you've got a bunch of cut up props, save them to your library, and voila... you can load them whenever you like with ease. :D

One thing the tutorial doesn't tell you is how to select polys easily. If you're in the regular screen mode, you can only select the polys you can see. Sometimes that's what you want, because you don't want the ones on the other side. But sometimes, if you're selecting polys to cut them out, you want to grab them all, and props often have lots of little tiny ones or ones hidden inside tubes or columns that you won't see until you've made your prop and there's a whole mess of squiggly bits and pieces still left that you meant to get rid of. In those cases, use the top most wireframe view. In that view, you can select all the polys with the marquee function within a rectangle, regardless of whether you can see them inside that rectangle or not. By alternating between views, you can select or deselect with perfect precision. But it does take practice, and is infuriating as holy living hell at first. Then once you've got it, it's a fantastic thing to be able to do.

If anyone knows any other tutorials on the grouping tool, they would probably be helpful, and I wouldn't mind seeing them myself. :)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.