There are various deductions allowable, such as the initial purchase costs (notary, land registry, transfer tax etc.) so that will help reduce your total liability. The retention at the notary only applies to non-residents, since residents should declare their capital gains as part of their annual tax declaration - even if you are not working now, you should still do one, even if it's a zero declaration. A gestor will charge maybe €50 to do this for you, and can advise you in more detail about capital gains tax. You will need to do an annual declaration anyway in order to obtain a certificate of fiscal residency from hacienda, which you will need to present to the notary in order to avoid the retention in the first place.
Not too sure what the second part of your question means; do you mean how much will an abogado charge to handle the sale of your property? If so, that would depend I suppose on what exactly you want them to do. What do you want them to do (that an estate agent won't do, or you can't do yourself)?