AutoCad Lite - buy it. You'll have it for ever. You'll be able to exchange dwg or dxf files with other designers - lighting, heating, plumbing, fire safety, escape routes etc - they'll thank you for it. Easy to download dwg/dxf files from manufacturers of almost anything that goes into a modern building including chairs, tables etc.
Don't forget grey matter, pencil, paper and scissors initially. Moving around cutouts on a background grid is a good way to get started 'broad brush' before committing to detail in CAD.
On the other hand you might get a local freelance CAD technician to do the job under your directions for little more than the cost of the software.
Hmm, I see it's called AutoCad LT now, you probably wouldn't need to buy the latest most expensive version, although it is available as a 30 day free trial in 'selected countries'.