New ships and aircraft for the Navy could cost $18.8 billion to $26.7 billion in the next five years depending on how Congress resolves a widening debate on the long-range mission of the service, a new report concludes.

A primarily defensive Navy with a broader mix of weapons systems than now exists would come with the lower price tag.

But if the Navy is to have the capability to launch massive offensive strikes at heavily fortified Soviet naval bases in northern Europe and the Pacific, the higher figure would be required.

This is the conclusion of a Congressional Budget Office report showing how policy decisions on strategic alternatives can add or subtract billions of dollars from weapons procurement expenditures.

The report was released as Congress begins debate on fiscal 1980 budget requests that will have considerable impact on the Navy's direction.

Among the major requests submitted by the administration are a new conventional aircraft carrier costing $1.6 billion; a destroyer, outfitted with the ultra-modern AEGIS air defense system, costing $825 million; and 24 F14s and 15 F18s costing $1.7 billion.

At the same tiem, the administration, in effect, has made an important decision by asking only $22.5 million for vertical/short take-off and landing (V/STOL) planes that some experts feel are the wave of the future in naval strategy.

The report noted that "a congressional decision in favor of an accelerated V/STOL program will influence both aircraft and shipbuilding programs over the next five years."

Long-temr planning is crucial in naval procure? ment because some large combat and support ships operate for 35 years or longer, and these vessels, in turn, are only compatible with certain weapons systems and planes (such as conventional aircraft that take off and land on a long carrier deck).

Procurement decisions are considered pressing because the fleet's size has declined from more than 1,000 vessels in 1970 to fewer than half that today, while the Soviet naval threat has "intensified significantly" with the introduction of aircraft carriers.

One theory argues that there is no need to build an offensive navy at this time. In the Atlantic, this thinking goes, a defensive strategy would not require attacks on Soviet bases in the Kola Peninsula. Instead, the Atlantic sea lanes could be protected from attacks by Soviet long-range Backfire bombers. And instead of attempting to knock out Soviet Siberian naval hases, U.S. aircraft carriers could be moved into the northwest Pacific to defend the sea lanes in that ocean.

This defensive strategy would, among other things, make questionable the need for the nuclear-powered carrier added to the 1979 defense budget but vetoed by Carter.

CBO outlined four policy options for the fiscal 1980 through 1984 period, each with different prices and roles for the Navy.

A "defensive wartime/current peacetime" mission would involve building one new conventional carrier and adding 573 F14s and F18s, at a cost of $21.6 billion.

A "defensive wartime/altered peacetime" option would scrap the conventional aircraft carrier, sharply reduce the number of F14s and stress a strengthened V/STOL capability The cost would be $18.8 billion

A third option, "offensive wartime/current altered peacetime," would emphasize V/STOL while building a new convntional carrier and giving the Navy the 220 F14s it wants, at a cost of $26.7 billion.

The final option, "offensive wartime/current peacetime," would give the Navy a nuclear carrier and a full complement of 220 F 14s, while postponing the introduction of V/STOL ships and planes.CBO priced this at $25.7 billion.