A raised bed of 8 to 12 inches high is plenty. unless you need the height due to a handicap and need to garden from a wheelchair.  The thick heavy roots on roses are anchor roots, The fine hair-like feeder roots stay within the top 2 to 5 inches of soil. We are often cautioned about not scratching the soil around the roses too deep for this reason. Lots of organics mixed with some of the soil you have will make a good mix. Compost, shredded leaves, aged horse manure, alfalfa meal, spagnum peat moss, any of these work well mixed in with your present soil. If you don't have enough soil to mix with the organics get a load of topsoil. A good additive for drainage and to keep the soil from compacting is perlite. You can mix it in up to a sixth of the volume if you wish. Although it does not add any nutritional value to the soil it does aid in water retention. Don't add any actual fertilizer until after the roses are planted and have started to set buds. The other things will suffice. If you have the patience to keep it together, a raised island of soil will work. You don't need any timbers as sides. Just raise the island 8 to 10 inches above the surrounding ground level. This requires some maintenance but it's not that hard to keep up. You can always add timbers if it gets out of hand.