Having been stuffed by a client recently and who will now have to pay me costs as well, for time involved in securing payment + court costs, I now operate like this.
Initial amount payable at the beginning. This is an amount that will cover my costs completely up to the point of delivery. So if the client reneges, I am not out money. (Breach of contract is a whole other ball-game for the loss of profit that would have been earned if they had not broken the contract plus costs (time) for the proceedings).
The purpose of enforcing payment means reneging is more costly for the client and is likely to discourage them but, it also puts on record who was in the wrong.
Where I live, such clients tend to spread the word that the tech was to blame for not being good enough when in most cases it is either a breakdown in communication or an absolute numpty of a client - as in my recent case).
On completion (imv when the project is finished and functional, ready to go live, and demonstrable to the client as a working entity), the balance becomes due. The balance includes my costs for any work involved in making the website live and operational and for any other ancilliary work involved. The costs payable at this stage include a retainer as well as any amount agreed from the outset or now, for support/daily back-ups etc.
I also recommend that the client is made aware from the beginning that any changes to be made from what was agreed at the first brief, will attract extra cost.
on building an ecommerce site from scratch I asked the client if they wanted a,b and c features. they stated that they wanted the site live first and would add those features later. I explained the extra costs of doing it later and explained why it was extra. (more time-consuming). they decided to have the added features done later. Now having just completed the thing, they want the other features. Having pre-warned them of extra costs, they know they have to pay them.
don't squash your costs down so as to put yourself under (time)pressure to complete. Better to make sure it works.
consider keeping your cards close to your chest. don't volunteer any discount early on. keep it until you can use it as a deal-maker or even (currently for you), as a form of added benefit to the client, in exchange for your building up a portfolio of work.
write every detail of the brief down and give a copy to them in writing as part of your 'order form'/contract. That way, in the event of a dispute, it's there in black and white for all to see and judge, if necessary.
make a note of your time during the process of building. this will keep you focussed on time management and also, provide a record for subsequent billing, say, if the client keeps ringing you up asking for you to 'build in this too that I saw at www.whatever.com
It takes time to see what they are talking about and to plan your costings for them. so you need either to charge for that or build it into your other fees.
As for length of time - I always overrun. build in a cushion of time. if someone else wants to do it quicker I just tell my (potential), client that I take the time to do it properly. If they want to pay someone to do it quickly, they may spend more money and time later, either getting it to work or to pay me to do it anyway.
you are just a person NOT a limited company. BUT you are a legally trading entity so that point is a bit moot, certainly, here in the UK. if you put yourself up as a whatever, you have to deliver just as if a limited company.
its called a sole trader in the uk.
payments: I agree the method of payment before giving a quote. If they are to pay on provision of the product I charge one amount. If they are to pay me wihtin 30 days, I charge another. BUT i also include the terms and conditions which explain what will happen in the event of payment not being paid on time. That can be daily interest or a fixed fee but if its fixed and they fall into the category of having to pay it, there is no incentive at that stage, to rush you the payment. so incremental additional costs work there.
And don't forget. you don't usually charge for your time in preparing the quote but, because it represents a cost to you, it should normally be included in your finished quote.