I'm afraid your sentence "das Haus war in 1845 gebaut" is not grammatically correct. It should read: "Das Haus wurde 1845 gebaut." If you wish to indicate that it was already standing in 1845, you would probably need to say something like "Das Haus stand schon 1845". While grammatically correct, I doubt whether the construction "Das Haus ist 1845 gebaut worden." would apply in terms of style, i.e. because it was built so long ago.
Also, the imperfect ("Präteritum") does seem to be used more commonly in northern Germany than here in the south.
I used to live in Oftersheim from late 1991 to December 2007. It's a dormitory town with about +/- 10,500 people but in good reach of both Heidelberg and Mannheim.
If you want somewhere a little bit bigger, you might want to consider Schwetzingen.
I didn't (still don't) have a car so had to depend on public transport (bus and train), which is quite good, e.g. to Heidelberg and Mannheim. However, if you have your own cars, that should not be an issue.
I don't know too much about Ketsch or Brühl but it strikes me that they might be a little to "far away" from anything else.
I don't know anything about real estate prices in all these areas but given the proximity to Heidelberg and Mannheim, they won't be cheap.
My conclusion: if you want somewhere small but not too small yet still "semi-rural" and with most facilities, I would recommend Schwetzingen.