It's generally a good plan to start off with something that sells readily at your local mart - either a 'white' commercial type, such as a Texel, or whatever is prevalent in your area. Spend a morning or two at the local primestock mart and see what's selling. Ayrshire - I'd guess locally prevalent breeds could include Scottish Blackface, Scots Mule, possibly Cheviot. Hopefully others from your area might chip in with better local knowledge.
Also you need to think about when you are wanting to buy and to sell - for instance, Texel-type stores will probably be available from June/July-ish, Cheviots and Blackies probably not until late August. Texels will probably finish on good grass with a little cake before the back end, Cheviots and Blackies will need some or maybe all of the winter to reach their slaughter size and condition - and will need cake if you are wanting to sell them before the spring grass comes in.
Another factor is how many you are wanting. At our marts, the hill types sell in very large batches (20 is a very small batch, 40-50 is more common) and it can be difficult to buy pens of less than 25. If this is the case in your mart, you may need to buy direct from a farm, in which case your local friendly farmer is your best bet, as s/he will hopefully be prepared to offer advice and even a little help, having sold you some store lambs.