For older style coaching stock you could look at some of the old 1970s Grafar/Graham Farish coaches. These have a 1920s look to them and are around a scale 55 foot long in both corridor and non-corridor formats. The old Triang/Hornby short Clerestory coaches can be hacked into all kinds of pre-group stock. A little more upmarket the old Mainline LMS period 1 corrdor coaches or the Airfix/Dapol LMS non-gangwayed lavatory stock are pretty good. While not scale, the older Hornby Gresley coaches have nicely moulded panelling and may lend themselves to chopping around. There are also the old Ratio (now under the Parkside brand) kits, 4 wheel and bogie, GWR, MR and LNWR though the bogies have a reputation for being a bit delicate.
With careful shopping around none of these options should be expensive.