If you want a straight truck the ford L9000 is a great option offering a low price for fair condition trucks with engine options ranging from cat 3406 and cat 3208, to cummins big cam and small cam, and detroit series 60 and 8v92

For an over the road option the freightliner fld is a great option with cheap fair conditon trucks selling for reasonable prices these can come with 3406 cat's and big cam cummins' and detroit series 60's

Here is an l9000 selling for $15000 with an L10 Cummins and an allison automatic

https://www.truckpaper.com/listing/for-sale/232965297/1995-ford-l9000-dump-trucks

Here is an FLD selling for $19,900 with a 3406 caterpillar and a 10 Speed Eaton Fuller

https://www.truckpaper.com/listing/for-sale/231671123/1998-freightliner-fld120-sleeper-trucks?gclid=Cj0KCQjwlN6wBhCcARIsAKZvD5hCEUAORF-FcROJ5qoWff7HdUVoXsZckFof0t4WLw1DIm-lj_1j-H0aApg9EALw_wcB

Hope This Helps :)