You cannot kill all the wild grass with weed killer only once. The seeds will grow and take over whatever new grass you grow.

After applying the weed killer and remove all the dead grass, loosening the soil, on the 2nd week, you should encourage the seeds to grow showering it with lot of water everyday for a month. Then kill the young wild grass or whatever trying to grow with weed killer again.

The next thing to do is cover the ground with a thin layer of sands, the sands will level the ground nicely without hardwork. When it rains, the rain water will wash the sands to low areas, you will see that areas that have no sands that are exposed you can then re-level the spots that were uneven easily.

Patient is the key to success. Try to kill as much of the wild "hidden seeds" by getting them to grow out of the seed/hide, in the long run you have less work pulling the young wild grass one by one.

River sand, the yellow color one use for plastering that have no stones. You can also use the white color one, use for reinforced concrete, which is residue of rocks and stones but it has to be sieved out the bigger pieces of stones. River sand is better as it contain mud.

For 1500sqft, one ton lorry will do. One ton lorry is the smallest lorry you find on the road, a little bigger than van, filling up to its side wooden panels and not overflow these side and back panels. It should fill up some 1 to 4 inches leveling the ground. The idea here is to level the ground that is to achieve at least 1 inch (high areas) and 4 inches to lower areas. It is easy to shift sand and you can never shift earth unless you use heavy machinery which in turn compacting the earth as it roll over and make the surface uneven even more for small areas like our houses compounds. If your ground is very level you use less sand and very uneven you use more. So, try to level the area that is too obviously uneven before sand. Not to worry trying to achieve perfection, it will self level as you walk on and rain, heavy rain. The best practice is to have a little slope so that water flow to where you want them to flow, ie to the drainholes/drain. You can use leveling spirit attached it to a one meter ruler making larger area leveling.

For big volume you buy directly from shop that specialized in selling sand, cement, stones and wood. If you cannot find one in your area, you can order from hardware shop and pay a little more.

There is no need to fertilise grass. What grass need most is water. Too much of fertilizer kill them and turned brown. What more the grass is newly planted that need watering everyday. What is the use of fertilizer when you have to water your newly planted grass everyday washing away the fertilizer? Got what I mean. See, your newly plnated grass need watering everyday and need fertiliser but you can have only one because water washes away fertiliser. Dont waste your money on fertilizer even when after the grass take root.

When you have a layer of sand, the grass spread faster as it is loss easy for it to reach new territory and the root will grow deep into the sand till it reached that earth. Because sand allow water to flow you will have less stagnant water/pool of water here and there. Too long stagnant water/flooding kills grass by destroying their leave that making it rot. Sand will also allows water to be lifted up for the grass as in capillary action, dont worry that the grass not getting water due to a layer of sand.

You can spread with anything when it is dry. Easy is to use a piece of plank to spread some 600mm to 1meter long. Use spirit level tie/stick to this plank to level. Just be sure when you pour, pour evenly make sure it is not too thick/ too much on one area.

Since you already have a level ground there is no need to add sand, moreover it is slope to drain. The new added sand will self-level when it rain heavy and your existing slope will disappear.

It is not advisable to add sand when you have planted the grass. You are supposed to add before planting so that it provides faster grow and improve underground water flow, avoid flooding and stagnant water that submerged the grass and kill it. With sand you do not need underground piping for draining unless you want to build high ground like a golf putting ground that have to use soil to hold the ground and not sands.

When you have grass grown you can never spread the sand evenly. You cannot see where you have spread and where is lacking and you cannot take it out under grass if you have spread too much on one place.

If you still want to have a layer of sand you should remove all the grass piece by piece if they have not taken root and leave it on some other places like your car porch that received some sunlight with old newspaper underneath as thick as possible and water twice a day lightly, care not to loose the soil and too water wash away the soil. You must not put one on top of the other, the one at the bottom will die. Alternatively, do one part at a time like one third. Diving it into 3 parts. Take part 1 out put it on a place like your car porch, sand and level part 1, take out part 2 grass and put/plant it on part 1 area, then take part 3 grass to part 2 area and finally plant the part 1 fr the porch to part 3 area. With the car porch occupied, I do not know where you are going to keep your sand unloading from lorry???   Alternatively, buy half lorry load at a time put it outside house compound, talk to immediate neighbour if you can use their compound outside their house for few days. make sure you have a layer of plastic sheet underneath otherwise you will leave a lot of sand on your neighbor compound. with plastic sheet you lose little of the sand.