How about just convert a civilian Ro/Ro vessel and add heli-deck/hanger + required military radar (just basic air and sea search) and communication systems. Like this one: http://en.wikipedia.org/wiki/HMNZS_Canterbury_%28L421%29
You probably only need a 76mm or a 25mm auto gun for this ship as 50% of the cost of a modern frigate is from the weapons and the sensors required for it. If you "build in but not fitted with" too much advance weapons (HF2, ASW system) in the design, the cost of the frigate will soon rise to be as much as a real frigate....... For other "advance" duties like escort, ASW, mine sweeping....etc, just receive "battle picture" via data link and let the helicopter (S-70/SH-60) or UAV/UCAV to do it.
小貓不會作模型,小貓只愛玩模型;看似小貓玩模型,其實模型玩小貓: A B 補土白膠條,樹脂改套蝕刻片;凹線凸線凹凸線,乾磨水磨銼刀磨; 水性油性消光漆,乾掃手掃噴槍掃;模型一盒三百幾,零件一堆三千幾; 多買多作進步多,買了不作口水多;花錢耗時傷精神,小貓是隻敗家喵。