Sorry to hear she didn't make it I'm afraid I've very little experience with spawning females, so I'm not really sure what could have been the complication. If she was releasing the eggs that's a good thing, often they need a male but not always and it's better for them to release them than not.
Sounds like you did everything possible though so try not to feel too sad, she did have a good life while she was with you
Smelts in the City - citizen science opportunity in London
If anyone in the London area has a bit of spare time and fancies being involved in a some fishy conservation there's a new citizen science project starting up :) I met Joe Pecorelli during the ZSL eel monitoring in the Thames and I can assure you that watching paint dry would be interesting if you had Joe to talk you through it, this is a man whose enthusiasm for aquatic life is infectious!
If you're referring to the white spots on the gills then the others are right and they are indeed breeding tubercles They come and go depending on what condition and how 'in the mood' the fish is. He looks quite young, so he won't be ready to do much about it yet but that won't stop him from feeling frisky and trying his luck with the ladies (or even the boys if he can't find a lady!). He looks fine and dandy to me but if you could give a bit more detail as per maccy_g's post that would help
Have you decided on what fish you want yet? To a certain degree that will influence the maintenance regime and decor as different species will have different requirements. Some species will benefit from an airpump, but others aren't especially bothered. They're nice to have though and can help circulation of water and prevent dead spots where the circulation is limited by decor etc.
But on the whole, your plan sounds about right
Once the fish are in it's wise to test the water weekly, just before you do a water change; to keep an eye on things change during the week. You'll also be able to judge if the water change needs to be a bit more depending on what's happening with the nitrate level.
If you're in a soft water area it would be a good idea to test for GH and KH and monitor that too. KH (carbonate hardness) is used up during the nitrogen cycle process and lack of carbonate hardness can stall the cycle and prevent the filter bacteria from doing their job. It can also cause a 'PH crash' whereby the water runs out of the ability to keep the PH stable as the buffering capacity is used up, the PH can then 'crash' becoming dangerously low and potentially killing the fish. Not nice, so definitely worth keeping an eye on! There's more info here which gives a nice explanation: http://fins.actwin.com/mirror/begin-chem.html
I used to have two heaters when I had the conventional 'in-tank' style ones, I found it was better than one. I had two 300w ones for my 350 litre (primarily because they came with it!) and then the same when I upped the tank size to 600 litres. I also tried the Rena Smartheaters that fit over the intake of the filter pipe, but I wasn't impressed with those. I've now got thermofilters and those are much better than any standalone heater I've tried, I wouldn't go back now
But going back to your original question, I'd go for two heaters on the back of the tank equidistant from each other and the sides. If that makes sense. That should help stop hot/cold spots as long as there is good circulation in the tank Plus if one fails then there other is still there.