Hola - as an ex rental agent, in the old days agents used to request a full month as a finders fee. Nowadays many do not charge anything (too many on the market) or charge as you see 50%. Its a catch 22 cos if its a place you really like, the agent is good, things get repeired quickly and the property itself may be at a negotiated down price, then it may be worth it. Be assured they will be etting another full month or yearly management fee from the owner and if you are a seemingly good tenant then you may be able to negotiate this out of the contract. Otherwise you will have to pay it!
If there is a genuine lack of rentals in your area of choice then you seemingly have no choice, but check with local papers first as at this time of the year everywhere is quiet!
Re the contract, it should be for min 11 months, there should be nothing nasty in it - mine was in both spanish and english so ask for a tranlation. You should have 15 days to report anything not working or worn etc and it should be repaired. The bills you pay are only elec, water gas if you have it. NOT the ibi (rates) or basura or community fees, unless you come to an agreement about the rent in which case thats up to you! Usually a standard contract and nothing to worry about. In it will have your notice to quit on the anniversary of the 11 month after possession - and usually as long as everything is ok you will be able to stay another period.
Changing locks - the landlord wont pay for that but you can do it yourself - you must ensure however that if access is needed for repairs, you give it - normally workmen make an appointment with you. Landlord has no right of entry without your poermission, sometimes this is in the contract and you can query it.
Have fun!