A few years ago I bought a Trek 950 mtb off CL for $75. I wound up putting slick tires on it since it never really leaves the pavement. It is a friggin' tank. Its heavy and clumsy.... and as far as I can tell, damn near indestructible. It mainly sees service pulling my daughter around on an In-step kids tandem. I'm around 270 right now and she & the tandem probably add an extra 65-70 lbs. That old trek just keeps on truckin'.
I guess my point is to not rule out an old Trek MTB for your stated purpose.