Did you mean 12000, not 1200? As far as used diesels, would stay away from the 6.0 and 6.4L Fords, troublesome history with these. Early Duramax GMs have had injector and head gasket problems. Dodge/Ram--not familiar with them. In a used diesel truck, previous owner care important if that can be determined, but still will be a risk in any of them. If the fiver is actually 12000 and under, possibly a Ford V10 gasser would work.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired