Well, we shouldn't be too pessimistic
Europe is based on ICAO, and generally the rules and regulations are more or less the same. Although there are indeed some differences, they shouldn't be a problem.
If you are flying IFR (I assume you mean Instrument Rated), then you should have no trouble. All agencies you talk to should be able to converse in English - although the quality (and accents) may vary considerably. How do you suppose the big boys who fly IFR all the time, manage?
In accordance with ICAO, the national rules require all controllers to be able to speak English. The R/T is standard ICAO. However, at small local airfields (particularly France), this is not always adhered to - but that will never be within controlled airspace.
Weather is indeed variable, depends where you go, although if you are IFR this should be less of a problem. If you are flying VFR, then you do have to plan carefully - you can get stuck in a place for days, waiting for the "clag" to lift.
It ain't nearly as cheap as in the States. Hiring a C172/PA28 for an hour is about 130 USD. It does vary from country to country, so you can add/subtract 15% of that depending where you are.
Currencies: the Euro is now in use in 12 European countries (Ireland, France, Belgium, Luxembourg, Netherlands, Germany, Italy, Austria, Spain, Portugal, Finlnd and Greece). However, not the UK. So you can use one currency for most of Europe but you will have to use Sterling for the UK. The Euro is more or less equal to 1 USD, at the moment.
Most of the European countries which form the European Union, have an agreement (called the Shengan agreement), whereby the customs and border requirements have been eased (i.e. free movement between countries). However, the UK is not one of them!
Cross border flying is no problem - you will have to file a flight plan though (if IFR, no problem).
If you can, go to the navigation office of a large airport and flip through the AIPs for several countries - terribly boring, but it should help
