Professor Michael Clarke, director general of the Royal United Services Institute think-tank, said Scotland could defend itself for between £2 billion and £3bn a year.
He said newly established armed forces could become "extraordinarily good" by 2030.
However, he warned a Scottish military would have to rely on more basic technology, with the Hawk trainer becoming the mainstay of the air force and the navy relying on second-hand Type 23 frigates surplus to Royal Navy requirements.
And he said most existing service personnel would want to remain with the UK's forces.
Mr Clarke presented the most detailed assessment yet of how the nation's armed forces may shape up in a lecture at Glasgow University organised by the Stevenson Trust for Citizenship.
The SNP envisages a £2.5bn defence budget to run a force equipped with a share of UK hardware, including fast jets.
Mr Clarke suggested a Scots navy would consist of about 2000 personnel and 20 vessels including minesweepers, coastal patrol vessels and three or four Type 23 frigates, which might be from the Royal Navy as they are to be replaced from 2021. It would cost £650 million per year to run.
The airforce – costing £370m to £400m per year – would have around 2000 personnel and about 60 aircraft.
The Scottish airforce would be equipped with maritime patrol planes and possibly two C130 transporters. The bulk of aircraft would be helicopters.
On the ground, Mr Clarke envisages a brigade of 8000 troops plus a larger territorial brigade of 12,000. The force, costing £900m to £1bn to run, would include army helicopters, artillery, medics and engineers.
Mr Clarke said such a military would be the "minimum force Scotland would want," allowing it to defend the country and contribute to combined operations with other countries.
A bigger version, costing between £2.5bn and £3.1bn, would include SAS-type special forces and units for areas such as nuclear defence.
He said that, even with £3bn, Scotland would have to buy in services such as training and repair and maintenance of equipment, probably from the UK.
He ruled out advanced equipment such as the Type 26 frigate which is due to replace the Type 23, the Typhoon fighter-bomber plane which entered service a decade ago, and the Joint Strike Fighter, planned for the UK's new aircraft carriers.
He said: "Scotland would have to accept top of the range boys' toys would be out of the question."
He added: "The prevailing view is if Scotland wanted to recruit forces of around 20,000 it would have to start from scratch. But there is no reason they could not be extraordinarily good by 2030."
His vision differs substantially from the SNP's defence policy.
The party promised a budget of £2.5bn to support a "multi-role brigade" of 15,000 regular service personnel and 5000 reservists. It claims Scotland would inherit existing Scottish-raised regiments and have special forces and Royal Marines. The policy says forces "will initially be equipped with Scotland's share of current assets such as ocean going vessels, fast jets for domestic air patrol duties, transport aircraft and helicopters as well as army vehicles, artillery and air defence systems".