I was in the same situation 2 years ago, ie. looking for affordable strobe with sufficient power for my SP-350. After lot of googling, I bought Epoque ES230DS strobe package c/w 10 bar flex arm tray and FO cable for just slightly above 300 bucks.
Although Epoque claim the strobe is GN23 (or GN20?), but It produce narrow beam - so the supplied difuser must be used all the time which certainly reduce the GN; though I have no idea the actual GN with the diffuser but enough to light up to 1.5 m at iso 200 f5.6 1/100. No complain since most of the time I shoot macro, For a strobe w/ 2 AA battery, the cycle time is not too bad.
I am not sure either the FO cable quality problem or the sensor not sensitive; I need to set the internal flash to max. to triger the strobe - sometime still fail particularly during bright day.
Having difficult time to shoot mandarin fish in Lembeh where my strobe was triggered by other divers strobe then I spend another 100 buck to change to epoque synch. cable (not TTL) and I am very pleased.
Epoque ES230DS can be considered as a budget strobe if you can get it at discounted price as mine (not at MRP) however I will not recommend the flexible arm - not flexible and too restrictive.
I also have Inon D2000 type 3 - much better strobe compared with epoque if you can increase your budget.
SP-350 is slow on autofocus; focus light will help much. I bought two focus light from Jack, ie. Fantasea 360LED and Bigblue which is more expensive but much better quality and stronger power but soft light. I attach the bigblue onto the PT-030 housing hotshoe - very convenience and sufficient for nigh dive as a torch. In fact I almost never use my main torch during night dive.
Cheers