Hello Tonz, and welcome.
With these mowers, the sky is the limit budget wise. But considering the economic climate right now, there are three things I'd get in order right away.
The first are the rail cracks. They can be welded and reinforced, or you can get them completely replaced over here in WA. Welded and reinforced rails will be the cheaper and quicker option but it won't last forever. I can't give you a cost there though as I've never fixed up a twin. Maybe budget $150?
The second is your clutch. I have no real idea what has been stuck in the oil hole, but I do know it ain't supposed to be there. I can only suspect it is holding things together? It may have damaged the PTO shaft and you should budget accordingly ($50). A completely new clutch half and inner cone could set you back $250. Trident Lawnmowers in WA makes new halves and SB Fabrication and Parts in SA does as well. I can only speak for Trident's workmanship (I have bought parts from both, but machined parts from Trident only) and it is top notch. Both have excellent reputations but their clutch halves are new to the scene. Regarding your thrust pad, plastic ($10) is just fine and cheaper than brass ($30). Both will wear from incorrect adjustment and lack of lubrication. Corks are cheap at around $8 for the clutch cone.
Finally, to my eyes, your cutter dual sprocket ($80) is well worn and should be replaced. The others look ok for now. I suspect your chain may be too tight here? You can buy a new sprocket from Outdoor King, as well as many other bits and pieces for the SB45 if you decide to purchase it and give it a good going over. You also have Reel Mower Solutions in QLD and CETE Parts (OPE Parts on eBay) in NSW. As well as your local mower shop...
The prices given are ball park but will give you an idea of what will need to be spent, plus the cost of the mower itself. See what you think, and once again, welcome.
