The Denon is a flawed product. Awful volume control with overly big increments, hiss/noise issues and badly positioned vents that allow dust to settle onto the CD tray. The first is a particular concerns in a bedroom where low volume levels are required.
The Marantz is a far better though-out product with a significantly better sound. If you move it to a lounge area later on it is also far more capable of driving much better speakers if the fancy takes you. Well worth the additional outlay if you can spend it.
£400 will buy a lot 2nd hand if you don't rush and choose carefully. My current system cost me £380. The amp completely out-classes any of the current budget amps and would compete quite comfortably with the likes of a Naim Nait 5i. The CD player is one of the most musical and involving ever made and the speakers have a level of build quality that doesn't exist much below £1k on the brand new market.
Of the three options, the Marantz will give you a great balance between excellent sound, features and compactness whilst the used seperates will give you the best all-out performance. The Denon option trails a long way behind IMO.