Cisco 3750 Gigabit ports

Firstly I'm a total believer that the larger Cisco switches are well worth the money IF you need them, if you don't and perhaps that's you then there's no point going for something of that cost. There's nothing wrong with the cheaper box but it will of course be missing some resilience and functions. Have you also considered HP's Procurve kit? I've not used it myself but there's a lot of people on this site that really rate it and it does come with lifetime support - might be worth a quick look.

Why continue to use media converters when the Power Connect 5424 supports fiber via SFP modules which are likely cheaper than the media converters?

1. The default configuration sounds like it should work out of the box for you. You'll only need Trunking between the switches if you plan on using multiple VLAN's. LAGS are useful if you want more throughput than one link can provide or are looking to improve the network availability. If you're running fiber between the switches this would mean multiple fiber runs.

2. This should be the default out of the box config. I know that older PowerConnect switches default to having STP enabled which can cause problems for some devices. So if you have a device that's failing to negotiate with the network appropriately you'll want to either disable STP or enable fast link on that port.

3. No other option really. The non-managed switch isn't going to support STP or LAGs.

Beyond that if you are using STP make sure that you manually set the STP priority on your central switch rather than leave it up to chance, normally based on the lowest MAC address, to determine the root STP switch. You may want to consider is connecting the 2 edge 5424's with a fiber link as well creating a loop. But if you do this make sure that you're using STP to prevent problems on the network. This will allow the network to reroute traffic around a broken link between the central and an edge 5424. This would be cheaper than running 2 fiber lines to each switch and use LAGs for redundancy and also has some advantages as the runs will most likely be in different physical locations. If it's in the budget you can always do both LAGs and this STP idea. You'll just need more fiber and more SFPs for the switches.