If you are considering using aluminium windows then ensure they are thermally-improved or thermally-broken frames, standard aluminium windows conduct a huge amount of heat into and out of your building, decreasing comfort and increasing running costs. Timber windows and thermally-broken aluminium windows are similar in terms of thermal performance. Well painted timber windows should be maintenance free for 10-15 years, yes they will require more effort over time than aluminium but it is not a weekly event. This is still a major factor for many people in making this choice. Accoya timber (have a Google) is relatively new but does appear to allow a painted finish to last even longer. Double glazing, yes it costs more but not twice as much. If you buy good quality frames why would you not also buy good quality glass (and vice versa). It can be relatively easy to replace the single glazing in your existing frames with double glazing although this depends on the frames themselves. These are probably the largest areas of glass in your home, if you use single glazing you will lose more than 50% of your heat through them (depending the level of insulation in the remainder of the building, possibly more) hence choosing the right glass is incredibly important. Building comfort is a product of many many factors, each decision contributes to the whole. Be mindful of 'death by a thousand cuts'. I hope this helps a little.

Years ago one of the tv diy shows made a wooden frame that sat in front of the window. It attached to the surrounding bricks and didn't attach to the window at all. Looked good from a distance on tv. I also stayed in a holiday flat where they'd been painted a charcoal colour. They looked great. I can imagine that they were fiddle to do.

About your question about sourcing some of your own internal fittings. It is best you discuss this with the builder early on. Usually, the builder's contract will specify an allowance for particular fixtures. However, some may specify actual brand and model of fittings ie taps, toilets, sinks, etc in their quote, and they may or may not allow you to vary this. IF you are happy with those specified fixtures and you think the allowance amount is reasonable, then you can go with it. Otherwise, any changes or variances need to be discussed with builder, and it is very important this be discussed upfront and before any contract is signed. Some building contracts may also disallow some variances. In any event, carefully read the contract so you completely understand what you are agreeing to, and discuss any and all parts where you are not in agreement so these issues can be ironed out. Also, and this is my own experience and opinion, but I advise specifying in the contract the completion date AND if the build is not completed on time then what happens (consider that you may be paying interest on your mortgage, be financing a bridging mortgage, etc. and/or you may also have already sold your home and then have to pay rent until the build is finished). If you are in agreement with builder to source some of the internal fittings yourself, there are any number of suppliers and sources to investigate, and this might be a good way to keep your budget in check. There will be Houzzers who are experienced at this and can provide you with ideas. Just keep in mind that you will have to have all fittings READY and ON SITE when the tradesmen require them, otherwise you will be holding up the build and that is going to cost YOU money.

I love that you're looking to invest in restoring it. I initially would have wondered whether there's still risk of it being bought by developers when you put it on the market again as they may retain the existing home to appease the heritage overlay but build others on the large block, especially if the area if growing in demand. In that case I would recommend keeping it simple, retaining cladding, just doing cosmetic changes plus fresh but inexpensive kitchen & bathroom. However, after reading your follow-up post, it sounds like you're actually going to be doing a lot with it and the integrity & heritage of the home is important to you, so I would do it well to attract a premium buyer, pricing it out of developers' budgets. In that case, avoid aluminium windows - even the more traditional style ones don't look quite right. And windows make such a statement visually so I think it would be ideal to invest in original style ones. My mum just did a renovation on a 1920s Tudor home and the windows that had to be replaced she ended up getting cheaper custom made by a local joinery than by the big Stegbars etc, but she did shop around. Alternatively there are building materials vendors/recyclers that could have good original windows, or even demolition homes listed on gumtree can come up with some great finds providing you're happy to remove yourself. And you need quite a few, I wouldn't worry about not having them identical from room to room which may be an issue finding so many matching, provided all your trims are painted say in a fresh white, they should tie in nicely together. And again, I was originally going to suggest keeping the cladding if you're doing a simple renovate to sell, however seeing all the work you're doing to the rest of the house, I'd be tempted to remove it and uncover the weather boards beneath. A once beautiful double-fronted Victorian house near me (unlike yours it was a complete dump - holes in the floor, had been derelict...) with vinyl cladding was sold last year and the new owners have removed the cladding and there was beautiful and very well preserved brick-pattern weatherboard underneath (I can't remember the name of it, it may not be common in your region). So you could have well preserved weather boards underneath. And if not, if you're both handy (and have trade friends) it is fairly straightforward and relatively inexpensive to replace them. And with your extension you'd want the weatherboards to all tie in too. It may also be easier and cheaper with wiring and insulation/sisalation too? And lastly, just make sure there's lots of light coming into that central lounge room from the adjacent rooms. Those features are amazing! Good luck! I'd love to see the progress photos!