You can get your heads decked, HGs replaced and fluids replaced in the states anywhere from $1200-2300 depending on where you go. This isn't a hard and fast rule. Do it yourself and save a some $$$.
Watch your temp gauge and check your fluid religiously and you might be able to get by for quite a while, although this isn't a recommendation.
As far as the shop saying you will only leak externally think about this; you are getting oil/hydrocarbons in your coolant overflow tank. Doesn't sound right to me.