Hi Ankit...

1) Yeh i suggest you model the items seperaetely and save them as .mdl. That way you can dynmically load them using scripting, which can increase fps.

2) For doors to open when a player is near them, you will need to use .mdl and then you can apply actions to make them move.

3) Its not wise to model everything all together. You may have some problems with the UVs as 3DGS only supports 1 UV set currently. Also you will take advantage of the BSP scene management which will create LOD and other scene rendering tricks.