The capital upper savings limit is £16k for most means-tested benefits, apart from Pension Credit, where there is no upper limit. Not all benefits are means-tested though, you can get contribution-based JSA or ESA regardless of income for a limited time, plus DLA is not means-tested at all. If they get child tax credits, this has a different way of assessing capital - savings don't affect your claim but they'd include interest from the savings in the claim. Carer's Allowance is also means-tested on income but not savings. It's not clear what benefits you mean by 'living on benefits' and the different benefits have different rules.
There are rules on deprivation of capital if they did something like spend money on luxury items. If they decide that they deliberately deprived themselves of capital in order to be eligible for benefits, the DWP would simply assess the couple as still having those savings.
The DWP/HMRC are generally only interested in cash type assets - bank accounts, stocks, shares, second properties, national savings. They don't ask about personal possessions like art or jewellery. In most cases they don't include the value of the home you live in. So if they were reported for fraud for having expensive cars which they'd accumulated before becoming eligible for benefits, it might well trigger an investigation but if they had evidence to show when it was bought, I doubt there could be a successful prosecution.
See the Decision Maker's Guide on Capital:
"Personal possessions such as clothing, jewellery, and cars are disregarded indefinitely (JSA Regs, Sch 8, para 15; IS (Gen) Regs, Sch 10, para 10)"
But:
"For JSA personal possessions are not disregarded if people buy them to reduce the amount of capital so they can get JSA or IS or more JSA or IS. For IS personal possessions are not disregarded if people buy them to reduce the amount of capital so they can get IS or more IS2 (see DMG 29807)."