For a cheap starter, under £50, you could try the Shakespeare Lincoln - £36 from here:
http://www.nimpopo.com/acatalog/coar...g_tackle_5.htm
I haven't fished with this model myself, but have had a good play with one in the shop and although it wouldn't win any beauty contests, I was amazed at how free running it was for a cheapo. It has a side-cast function, which some people like, but irrelevant really for trotting and side casting can put twist in your line over time.
I have seem some very cheap pin's, as in a tenner, on e-bay but I cannot believe they will run freely or truly enough for trotting. I am still mainly using a Leeds Classic (Lewtham engineering), which can be a b*gger for line getting behind the spool, but I am still rather fond of it because its a bearings reel and spins for ever, also has an optional click check, which can be useful if using it for margin carp fishing. You can pick those up on e-bay reasonably cheaply sometimes. Sometime I will replace it with an Okuma Sheffield, which seem very good for the new breed (another bearings reel, which I prefer to a running on an actual centre pin). It can be a bit of a lottery on ebay, because if they've been dropped or knocked around a bit, they can aquire a bit of a wobble, so if possible I would try and find one locally and check it out first before buying.