suprised to see no replies yet. you should probably also mention your weight and local conditions as this is pretty important to help choose a board size. generally i think the best thing to do is too see what the good sailors at your local spots are sailing, this will give you a good idea of what to look for.
the general view is that a slalom board is a faster more performance oriented board which is also more technical AKA difficult to sail. I wouldn't recommend one as a first board. freeride board is a more general use board, they are usually easier to sail, and a great option for an intermediate. neither are really suitable for sailing in the surf thou.
a lot of people these days also recommend the freestyle wave type boards as they allow you to do a bit of everything, they generally are even more maneuver orientated then the freeride boards, and are designed to be used in the surf as well as flat water. personally i reckon for flat water a pure freestyle board is a much better option.
second hand market is pretty good, as long as you have some patience. if its within your budget a good benchmark you will look to be paying around $1400 or less for a pro/team-edition(expensive construction, lighter and stiffer but a bit more fragile) 2009 board in really good condition, and a bit less for a "standard" construction board. anymore then that and you will definately get a better deal with some patience.
for a first board back its probably best to look for something around 2007/8 vintage , you can probably find board in good condition for under $800. For example if you were a 70-75kg intermediate who can water start and has sailed 80L boards before a good lighter wind board for you might be something like this:
www.seabreeze.com.au/Classifieds/Windsurfing/Boards/~rzq_9/2008-JP-Australia-Freestyle-Wave-101-litres.aspx?search=eLRMcKwwdw7twdE4rW/yEaV/dIR/0dwb i'd offer then $800, and go from there. thou i personally reckon freestyle wave boards are rubbish, but a lot of people swear by them.
lastly you can buy boards from $300-500 if you are looking at something a fair it cheaper, and there are a lot of good boards that age, but you do need to be careful as i'd say boards are definately a 'consumable' item, they generally don't get better with age.
i've probably typed enough already, but i'd say if possible, invest a bit more in your rig first up and spend a bit less on the board. a good modern light rig can make a massive difference to how easy your kit is to sail.
