I've been growing veg here pretty much for the last 3 years. Chicken manure can be a bit too rich at times - you need to leave it for a year (so I'm told!). On the other hand, rabbit manure can be used immediately.  

You can buy seedlings from the indoor market in Tortosa (2 stalls sell them) - they are spanish varieties so well suited to the climate - I've tried some english varieties with poor success. Prices are very reasonable - a few cents each for lettuice, peppers aubergines etc. A large bunch of onions (normally around 150 ish) costs around a cent per plant. Hardly seems worth raising from seed!

I had little success with english runner beans or melons, but good success with chillies, peppers, swede, cucumber, potatoes, raddish, marrow, aubergines, carrots and many other things (English strawberries do well if you can get them established).  I'm currently experimenting with rhubarb grown from seed - so far looks promising, just have to see how they cope with the summer heat.

You can get 2 growing seasons here if you time things right - spring and autumn.

Best tip, keep an eye out for what the locals are planting and what seedlings are being sold in the market. Also, if you really want to raise from seed, Lidl sell various seeds at reasonable prices.

Nigel