That cab will be perfect IMO. You can use the 250w HPS and I would get 4 42W CFL lights that are 6500K and mount them in the corners at about 20 in up from the bottom of the cab. I am not sure if you even know what CFL's are and what 6500k means, just ask, or do some research first and then you can ask more informed questions. Look at some more pictures and threads in the cabinet section and get an idea of what people are using and how they are using it. You need to think about fans, reflective material, light proofing and smell, just to name a few basics.
A rule of thumb to follow for lighting requirements is to get 50w's of light per square FT for optimal growth. With a 2x3 cab you have 6 Square FT x that by 50 and it = 300w. The idea that I am suggesting with the 4 CFL lights and your HPS will give you more than 50w Per Square FT and also provide you with a full spectrum that will allow you to do veg and flower in one cab. I am not sure what the fellow was talking about that said you couldn't do it or what the comment was based on unless I read something wrong. I think with a 250W and the CFL lights you could do all right for some personal stash depending on how much you toke and how dedicated you are willing to be in this learning process. Justaseed a grower on this board has pulled some nice harvests using a style called SCROG with a 250 set up. I think that he pulled over a QP, but he is an awesome scrogger who knows is stuff, the only reason I mention it is to let you know some of the options. Look up his grow titled "Lovely Ladies".
Adieu